ELF loader support for auxvec base platform string

Some IBM POWER-based platforms have the ability to run in a
mode which mostly appears to the OS as a different processor from the
actual hardware.  For example, a Power6 system may appear to be a
Power5+, which makes the AT_PLATFORM value "power5+".  This means that
programs are restricted to the ISA supported by Power5+;
Power6-specific instructions are treated as illegal.

However, some applications (virtual machines, optimized libraries) can
benefit from knowledge of the underlying CPU model.  A new aux vector
entry, AT_BASE_PLATFORM, will denote the actual hardware.  For
example, on a Power6 system in Power5+ compatibility mode, AT_PLATFORM
will be "power5+" and AT_BASE_PLATFORM will be "power6".  The idea is
that AT_PLATFORM indicates the instruction set supported, while
AT_BASE_PLATFORM indicates the underlying microarchitecture.

If the architecture has defined ELF_BASE_PLATFORM, copy that value to
the user stack in the same manner as ELF_PLATFORM.

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

2 files changed, 33 insertions, 1 deletions

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 639d2d8b5710..742c8f530481 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -131,6 +131,15 @@ static int padzero(unsigned long elf_bss)
 #define STACK_ALLOC(sp, len) ({ sp -= len ; sp; })
 #endif
 
+#ifndef ELF_BASE_PLATFORM
+/*
+ * AT_BASE_PLATFORM indicates the "real" hardware/microarchitecture.
+ * If the arch defines ELF_BASE_PLATFORM (in asm/elf.h), the value
+ * will be copied to the user stack in the same manner as AT_PLATFORM.
+ */
+#define ELF_BASE_PLATFORM NULL
+#endif
+
 static int
 create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
                 unsigned long load_addr, unsigned long interp_load_addr)
@@ -142,7 +151,9 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
         elf_addr_t __user *envp;
         elf_addr_t __user *sp;
         elf_addr_t __user *u_platform;
+        elf_addr_t __user *u_base_platform;
         const char *k_platform = ELF_PLATFORM;
+        const char *k_base_platform = ELF_BASE_PLATFORM;
         int items;
         elf_addr_t *elf_info;
         int ei_index = 0;
@@ -172,6 +183,19 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
                         return -EFAULT;
         }
 
+        /*
+         * If this architecture has a "base" platform capability
+         * string, copy it to userspace.
+         */
+        u_base_platform = NULL;
+        if (k_base_platform) {
+                size_t len = strlen(k_base_platform) + 1;
+
+                u_base_platform = (elf_addr_t __user *)STACK_ALLOC(p, len);
+                if (__copy_to_user(u_base_platform, k_base_platform, len))
+                        return -EFAULT;
+        }
+
         /* Create the ELF interpreter info */
         elf_info = (elf_addr_t *)current->mm->saved_auxv;
         /* update AT_VECTOR_SIZE_BASE if the number of NEW_AUX_ENT() changes */
@@ -209,6 +233,10 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
                 NEW_AUX_ENT(AT_PLATFORM,
                             (elf_addr_t)(unsigned long)u_platform);
         }
+        if (k_base_platform) {
+                NEW_AUX_ENT(AT_BASE_PLATFORM,
+                            (elf_addr_t)(unsigned long)u_base_platform);
+        }
         if (bprm->interp_flags & BINPRM_FLAGS_EXECFD) {
                 NEW_AUX_ENT(AT_EXECFD, bprm->interp_data);
         }
diff --git a/include/linux/auxvec.h b/include/linux/auxvec.h
index 0da17d14fd13..d7afa9dd6635 100644
--- a/include/linux/auxvec.h
+++ b/include/linux/auxvec.h
@@ -26,9 +26,13 @@
 
 #define AT_SECURE 23   /* secure mode boolean */
 
+#define AT_BASE_PLATFORM 24     /* string identifying real platform, may
+                                 * differ from AT_PLATFORM. */
+
 #define AT_EXECFN  31   /* filename of program */
+
 #ifdef __KERNEL__
-#define AT_VECTOR_SIZE_BASE 17 /* NEW_AUX_ENT entries in auxiliary table */
+#define AT_VECTOR_SIZE_BASE 18 /* NEW_AUX_ENT entries in auxiliary table */
   /* number of "#define AT_.*" above, minus {AT_NULL, AT_IGNORE, AT_NOTELF} */
 #endif