arm64: User access library functions

This patch add support for various user access functions. These
functions use the standard LDR/STR instructions and not the LDRT/STRT
variants in order to allow kernel addresses (after set_fs(KERNEL_DS)).

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Olof Johansson <olof@lixom.net>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>

7 files changed, 642 insertions, 0 deletions

diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
new file mode 100644
index 000000000000..008f8481da65
--- /dev/null
+++ b/arch/arm64/include/asm/uaccess.h
@@ -0,0 +1,297 @@
+/*
+ * Based on arch/arm/include/asm/uaccess.h
+ *
+ * Copyright (C) 2012 ARM Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#ifndef __ASM_UACCESS_H
+#define __ASM_UACCESS_H
+
+/*
+ * User space memory access functions
+ */
+#include <linux/string.h>
+#include <linux/thread_info.h>
+
+#include <asm/ptrace.h>
+#include <asm/errno.h>
+#include <asm/memory.h>
+#include <asm/compiler.h>
+
+#define VERIFY_READ 0
+#define VERIFY_WRITE 1
+
+/*
+ * The exception table consists of pairs of addresses: the first is the
+ * address of an instruction that is allowed to fault, and the second is
+ * the address at which the program should continue.  No registers are
+ * modified, so it is entirely up to the continuation code to figure out
+ * what to do.
+ *
+ * All the routines below use bits of fixup code that are out of line
+ * with the main instruction path.  This means when everything is well,
+ * we don't even have to jump over them.  Further, they do not intrude
+ * on our cache or tlb entries.
+ */
+
+struct exception_table_entry
+{
+        unsigned long insn, fixup;
+};
+
+extern int fixup_exception(struct pt_regs *regs);
+
+#define KERNEL_DS       (-1UL)
+#define get_ds()        (KERNEL_DS)
+
+#define USER_DS         TASK_SIZE_64
+#define get_fs()        (current_thread_info()->addr_limit)
+
+static inline void set_fs(mm_segment_t fs)
+{
+        current_thread_info()->addr_limit = fs;
+}
+
+#define segment_eq(a,b) ((a) == (b))
+
+/*
+ * Return 1 if addr < current->addr_limit, 0 otherwise.
+ */
+#define __addr_ok(addr)                                                 \
+({                                                                      \
+        unsigned long flag;                                             \
+        asm("cmp %1, %0; cset %0, lo"                                   \
+                : "=&r" (flag)                                          \
+                : "r" (addr), "0" (current_thread_info()->addr_limit)   \
+                : "cc");                                                \
+        flag;                                                           \
+})
+
+/*
+ * Test whether a block of memory is a valid user space address.
+ * Returns 1 if the range is valid, 0 otherwise.
+ *
+ * This is equivalent to the following test:
+ * (u65)addr + (u65)size < (u65)current->addr_limit
+ *
+ * This needs 65-bit arithmetic.
+ */
+#define __range_ok(addr, size)                                          \
+({                                                                      \
+        unsigned long flag, roksum;                                     \
+        __chk_user_ptr(addr);                                           \
+        asm("adds %1, %1, %3; ccmp %1, %4, #2, cc; cset %0, cc"         \
+                : "=&r" (flag), "=&r" (roksum)                          \
+                : "1" (addr), "Ir" (size),                              \
+                  "r" (current_thread_info()->addr_limit)               \
+                : "cc");                                                \
+        flag;                                                           \
+})
+
+#define access_ok(type, addr, size)     __range_ok(addr, size)
+
+/*
+ * The "__xxx" versions of the user access functions do not verify the address
+ * space - it must have been done previously with a separate "access_ok()"
+ * call.
+ *
+ * The "__xxx_error" versions set the third argument to -EFAULT if an error
+ * occurs, and leave it unchanged on success.
+ */
+#define __get_user_asm(instr, reg, x, addr, err)                        \
+        asm volatile(                                                   \
+        "1:     " instr "       " reg "1, [%2]\n"                       \
+        "2:\n"                                                          \
+        "       .section .fixup, \"ax\"\n"                              \
+        "       .align  2\n"                                            \
+        "3:     mov     %w0, %3\n"                                      \
+        "       mov     %1, #0\n"                                       \
+        "       b       2b\n"                                           \
+        "       .previous\n"                                            \
+        "       .section __ex_table,\"a\"\n"                            \
+        "       .align  3\n"                                            \
+        "       .quad   1b, 3b\n"                                       \
+        "       .previous"                                              \
+        : "+r" (err), "=&r" (x)                                         \
+        : "r" (addr), "i" (-EFAULT))
+
+#define __get_user_err(x, ptr, err)                                     \
+do {                                                                    \
+        unsigned long __gu_val;                                         \
+        __chk_user_ptr(ptr);                                            \
+        switch (sizeof(*(ptr))) {                                       \
+        case 1:                                                         \
+                __get_user_asm("ldrb", "%w", __gu_val, (ptr), (err));   \
+                break;                                                  \
+        case 2:                                                         \
+                __get_user_asm("ldrh", "%w", __gu_val, (ptr), (err));   \
+                break;                                                  \
+        case 4:                                                         \
+                __get_user_asm("ldr", "%w", __gu_val, (ptr), (err));    \
+                break;                                                  \
+        case 8:                                                         \
+                __get_user_asm("ldr", "%",  __gu_val, (ptr), (err));    \
+                break;                                                  \
+        default:                                                        \
+                BUILD_BUG();                                            \
+        }                                                               \
+        (x) = (__typeof__(*(ptr)))__gu_val;                             \
+} while (0)
+
+#define __get_user(x, ptr)                                              \
+({                                                                      \
+        int __gu_err = 0;                                               \
+        __get_user_err((x), (ptr), __gu_err);                           \
+        __gu_err;                                                       \
+})
+
+#define __get_user_error(x, ptr, err)                                   \
+({                                                                      \
+        __get_user_err((x), (ptr), (err));                              \
+        (void)0;                                                        \
+})
+
+#define __get_user_unaligned __get_user
+
+#define get_user(x, ptr)                                                \
+({                                                                      \
+        might_sleep();                                                  \
+        access_ok(VERIFY_READ, (ptr), sizeof(*(ptr))) ?                 \
+                __get_user((x), (ptr)) :                                \
+                ((x) = 0, -EFAULT);                                     \
+})
+
+#define __put_user_asm(instr, reg, x, addr, err)                        \
+        asm volatile(                                                   \
+        "1:     " instr "       " reg "1, [%2]\n"                       \
+        "2:\n"                                                          \
+        "       .section .fixup,\"ax\"\n"                               \
+        "       .align  2\n"                                            \
+        "3:     mov     %w0, %3\n"                                      \
+        "       b       2b\n"                                           \
+        "       .previous\n"                                            \
+        "       .section __ex_table,\"a\"\n"                            \
+        "       .align  3\n"                                            \
+        "       .quad   1b, 3b\n"                                       \
+        "       .previous"                                              \
+        : "+r" (err)                                                    \
+        : "r" (x), "r" (addr), "i" (-EFAULT))
+
+#define __put_user_err(x, ptr, err)                                     \
+do {                                                                    \
+        __typeof__(*(ptr)) __pu_val = (x);                              \
+        __chk_user_ptr(ptr);                                            \
+        switch (sizeof(*(ptr))) {                                       \
+        case 1:                                                         \
+                __put_user_asm("strb", "%w", __pu_val, (ptr), (err));   \
+                break;                                                  \
+        case 2:                                                         \
+                __put_user_asm("strh", "%w", __pu_val, (ptr), (err));   \
+                break;                                                  \
+        case 4:                                                         \
+                __put_user_asm("str",  "%w", __pu_val, (ptr), (err));   \
+                break;                                                  \
+        case 8:                                                         \
+                __put_user_asm("str",  "%", __pu_val, (ptr), (err));    \
+                break;                                                  \
+        default:                                                        \
+                BUILD_BUG();                                            \
+        }                                                               \
+} while (0)
+
+#define __put_user(x, ptr)                                              \
+({                                                                      \
+        int __pu_err = 0;                                               \
+        __put_user_err((x), (ptr), __pu_err);                           \
+        __pu_err;                                                       \
+})
+
+#define __put_user_error(x, ptr, err)                                   \
+({                                                                      \
+        __put_user_err((x), (ptr), (err));                              \
+        (void)0;                                                        \
+})
+
+#define __put_user_unaligned __put_user
+
+#define put_user(x, ptr)                                                \
+({                                                                      \
+        might_sleep();                                                  \
+        access_ok(VERIFY_WRITE, (ptr), sizeof(*(ptr))) ?                \
+                __put_user((x), (ptr)) :                                \
+                -EFAULT;                                                \
+})
+
+extern unsigned long __must_check __copy_from_user(void *to, const void __user *from, unsigned long n);
+extern unsigned long __must_check __copy_to_user(void __user *to, const void *from, unsigned long n);
+extern unsigned long __must_check __copy_in_user(void __user *to, const void __user *from, unsigned long n);
+extern unsigned long __must_check __clear_user(void __user *addr, unsigned long n);
+
+extern unsigned long __must_check __strncpy_from_user(char *to, const char __user *from, unsigned long count);
+extern unsigned long __must_check __strnlen_user(const char __user *s, long n);
+
+static inline unsigned long __must_check copy_from_user(void *to, const void __user *from, unsigned long n)
+{
+        if (access_ok(VERIFY_READ, from, n))
+                n = __copy_from_user(to, from, n);
+        else /* security hole - plug it */
+                memset(to, 0, n);
+        return n;
+}
+
+static inline unsigned long __must_check copy_to_user(void __user *to, const void *from, unsigned long n)
+{
+        if (access_ok(VERIFY_WRITE, to, n))
+                n = __copy_to_user(to, from, n);
+        return n;
+}
+
+static inline unsigned long __must_check copy_in_user(void __user *to, const void __user *from, unsigned long n)
+{
+        if (access_ok(VERIFY_READ, from, n) && access_ok(VERIFY_WRITE, to, n))
+                n = __copy_in_user(to, from, n);
+        return n;
+}
+
+#define __copy_to_user_inatomic __copy_to_user
+#define __copy_from_user_inatomic __copy_from_user
+
+static inline unsigned long __must_check clear_user(void __user *to, unsigned long n)
+{
+        if (access_ok(VERIFY_WRITE, to, n))
+                n = __clear_user(to, n);
+        return n;
+}
+
+static inline long __must_check strncpy_from_user(char *dst, const char __user *src, long count)
+{
+        long res = -EFAULT;
+        if (access_ok(VERIFY_READ, src, 1))
+                res = __strncpy_from_user(dst, src, count);
+        return res;
+}
+
+#define strlen_user(s)  strnlen_user(s, ~0UL >> 1)
+
+static inline long __must_check strnlen_user(const char __user *s, long n)
+{
+        unsigned long res = 0;
+
+        if (__addr_ok(s))
+                res = __strnlen_user(s, n);
+
+        return res;
+}
+
+#endif /* __ASM_UACCESS_H */
diff --git a/arch/arm64/lib/clear_user.S b/arch/arm64/lib/clear_user.S
new file mode 100644
index 000000000000..6e0ed93d51fe
--- /dev/null
+++ b/arch/arm64/lib/clear_user.S
@@ -0,0 +1,58 @@
+/*
+ * Based on arch/arm/lib/clear_user.S
+ *
+ * Copyright (C) 2012 ARM Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+
+        .text
+
+/* Prototype: int __clear_user(void *addr, size_t sz)
+ * Purpose  : clear some user memory
+ * Params   : addr - user memory address to clear
+ *          : sz   - number of bytes to clear
+ * Returns  : number of bytes NOT cleared
+ *
+ * Alignment fixed up by hardware.
+ */
+ENTRY(__clear_user)
+        mov     x2, x1                  // save the size for fixup return
+        subs    x1, x1, #8
+        b.mi    2f
+1:
+USER(9f, str    xzr, [x0], #8   )
+        subs    x1, x1, #8
+        b.pl    1b
+2:      adds    x1, x1, #4
+        b.mi    3f
+USER(9f, str    wzr, [x0], #4   )
+        sub     x1, x1, #4
+3:      adds    x1, x1, #2
+        b.mi    4f
+USER(9f, strh   wzr, [x0], #2   )
+        sub     x1, x1, #2
+4:      adds    x1, x1, #1
+        b.mi    5f
+        strb    wzr, [x0]
+5:      mov     x0, #0
+        ret
+ENDPROC(__clear_user)
+
+        .section .fixup,"ax"
+        .align  2
+9:      mov     x0, x2                  // return the original size
+        ret
+        .previous
diff --git a/arch/arm64/lib/copy_from_user.S b/arch/arm64/lib/copy_from_user.S
new file mode 100644
index 000000000000..5e27add9d362
--- /dev/null
+++ b/arch/arm64/lib/copy_from_user.S
@@ -0,0 +1,66 @@
+/*
+ * Copyright (C) 2012 ARM Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+
+/*
+ * Copy from user space to a kernel buffer (alignment handled by the hardware)
+ *
+ * Parameters:
+ *      x0 - to
+ *      x1 - from
+ *      x2 - n
+ * Returns:
+ *      x0 - bytes not copied
+ */
+ENTRY(__copy_from_user)
+        add     x4, x1, x2                      // upper user buffer boundary
+        subs    x2, x2, #8
+        b.mi    2f
+1:
+USER(9f, ldr    x3, [x1], #8    )
+        subs    x2, x2, #8
+        str     x3, [x0], #8
+        b.pl    1b
+2:      adds    x2, x2, #4
+        b.mi    3f
+USER(9f, ldr    w3, [x1], #4    )
+        sub     x2, x2, #4
+        str     w3, [x0], #4
+3:      adds    x2, x2, #2
+        b.mi    4f
+USER(9f, ldrh   w3, [x1], #2    )
+        sub     x2, x2, #2
+        strh    w3, [x0], #2
+4:      adds    x2, x2, #1
+        b.mi    5f
+USER(9f, ldrb   w3, [x1]        )
+        strb    w3, [x0]
+5:      mov     x0, #0
+        ret
+ENDPROC(__copy_from_user)
+
+        .section .fixup,"ax"
+        .align  2
+9:      sub     x2, x4, x1
+        mov     x3, x2
+10:     strb    wzr, [x0], #1                   // zero remaining buffer space
+        subs    x3, x3, #1
+        b.ne    10b
+        mov     x0, x2                          // bytes not copied
+        ret
+        .previous
diff --git a/arch/arm64/lib/copy_in_user.S b/arch/arm64/lib/copy_in_user.S
new file mode 100644
index 000000000000..84b6c9bb9b93
--- /dev/null
+++ b/arch/arm64/lib/copy_in_user.S
@@ -0,0 +1,63 @@
+/*
+ * Copy from user space to user space
+ *
+ * Copyright (C) 2012 ARM Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+
+/*
+ * Copy from user space to user space (alignment handled by the hardware)
+ *
+ * Parameters:
+ *      x0 - to
+ *      x1 - from
+ *      x2 - n
+ * Returns:
+ *      x0 - bytes not copied
+ */
+ENTRY(__copy_in_user)
+        add     x4, x0, x2                      // upper user buffer boundary
+        subs    x2, x2, #8
+        b.mi    2f
+1:
+USER(9f, ldr    x3, [x1], #8    )
+        subs    x2, x2, #8
+USER(9f, str    x3, [x0], #8    )
+        b.pl    1b
+2:      adds    x2, x2, #4
+        b.mi    3f
+USER(9f, ldr    w3, [x1], #4    )
+        sub     x2, x2, #4
+USER(9f, str    w3, [x0], #4    )
+3:      adds    x2, x2, #2
+        b.mi    4f
+USER(9f, ldrh   w3, [x1], #2    )
+        sub     x2, x2, #2
+USER(9f, strh   w3, [x0], #2    )
+4:      adds    x2, x2, #1
+        b.mi    5f
+USER(9f, ldrb   w3, [x1]        )
+USER(9f, strb   w3, [x0]        )
+5:      mov     x0, #0
+        ret
+ENDPROC(__copy_in_user)
+
+        .section .fixup,"ax"
+        .align  2
+9:      sub     x0, x4, x0                      // bytes not copied
+        ret
+        .previous
diff --git a/arch/arm64/lib/copy_to_user.S b/arch/arm64/lib/copy_to_user.S
new file mode 100644
index 000000000000..a0aeeb9b7a28
--- /dev/null
+++ b/arch/arm64/lib/copy_to_user.S
@@ -0,0 +1,61 @@
+/*
+ * Copyright (C) 2012 ARM Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+
+/*
+ * Copy to user space from a kernel buffer (alignment handled by the hardware)
+ *
+ * Parameters:
+ *      x0 - to
+ *      x1 - from
+ *      x2 - n
+ * Returns:
+ *      x0 - bytes not copied
+ */
+ENTRY(__copy_to_user)
+        add     x4, x0, x2                      // upper user buffer boundary
+        subs    x2, x2, #8
+        b.mi    2f
+1:
+        ldr     x3, [x1], #8
+        subs    x2, x2, #8
+USER(9f, str    x3, [x0], #8    )
+        b.pl    1b
+2:      adds    x2, x2, #4
+        b.mi    3f
+        ldr     w3, [x1], #4
+        sub     x2, x2, #4
+USER(9f, str    w3, [x0], #4    )
+3:      adds    x2, x2, #2
+        b.mi    4f
+        ldrh    w3, [x1], #2
+        sub     x2, x2, #2
+USER(9f, strh   w3, [x0], #2    )
+4:      adds    x2, x2, #1
+        b.mi    5f
+        ldrb    w3, [x1]
+USER(9f, strb   w3, [x0]        )
+5:      mov     x0, #0
+        ret
+ENDPROC(__copy_to_user)
+
+        .section .fixup,"ax"
+        .align  2
+9:      sub     x0, x4, x0                      // bytes not copied
+        ret
+        .previous
diff --git a/arch/arm64/lib/strncpy_from_user.S b/arch/arm64/lib/strncpy_from_user.S
new file mode 100644
index 000000000000..56e448a831a0
--- /dev/null
+++ b/arch/arm64/lib/strncpy_from_user.S
@@ -0,0 +1,50 @@
+/*
+ * Based on arch/arm/lib/strncpy_from_user.S
+ *
+ * Copyright (C) 1995-2000 Russell King
+ * Copyright (C) 2012 ARM Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+#include <asm/errno.h>
+
+        .text
+        .align  5
+
+/*
+ * Copy a string from user space to kernel space.
+ *  x0 = dst, x1 = src, x2 = byte length
+ * returns the number of characters copied (strlen of copied string),
+ *  -EFAULT on exception, or "len" if we fill the whole buffer
+ */
+ENTRY(__strncpy_from_user)
+        mov     x4, x1
+1:      subs    x2, x2, #1
+        bmi     2f
+USER(9f, ldrb   w3, [x1], #1    )
+        strb    w3, [x0], #1
+        cbnz    w3, 1b
+        sub     x1, x1, #1      // take NUL character out of count
+2:      sub     x0, x1, x4
+        ret
+ENDPROC(__strncpy_from_user)
+
+        .section .fixup,"ax"
+        .align  0
+9:      strb    wzr, [x0]       // null terminate
+        mov     x0, #-EFAULT
+        ret
+        .previous
diff --git a/arch/arm64/lib/strnlen_user.S b/arch/arm64/lib/strnlen_user.S
new file mode 100644
index 000000000000..7f7b176a5646
--- /dev/null
+++ b/arch/arm64/lib/strnlen_user.S
@@ -0,0 +1,47 @@
+/*
+ * Based on arch/arm/lib/strnlen_user.S
+ *
+ * Copyright (C) 1995-2000 Russell King
+ * Copyright (C) 2012 ARM Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+#include <asm/errno.h>
+
+        .text
+        .align  5
+
+/* Prototype: unsigned long __strnlen_user(const char *str, long n)
+ * Purpose  : get length of a string in user memory
+ * Params   : str - address of string in user memory
+ * Returns  : length of string *including terminator*
+ *            or zero on exception, or n if too long
+ */
+ENTRY(__strnlen_user)
+        mov     x2, x0
+1:      subs    x1, x1, #1
+        b.mi    2f
+USER(9f, ldrb   w3, [x0], #1    )
+        cbnz    w3, 1b
+2:      sub     x0, x0, x2
+        ret
+ENDPROC(__strnlen_user)
+
+        .section .fixup,"ax"
+        .align  0
+9:      mov     x0, #0
+        ret
+        .previous