MIPS: O32: Do not handle require 32 bytes from the stack to be readable.

Commit 46e12c07b3b9603c60fc1d421ff18618241cb081 (MIPS: O32 / 32-bit:
Always copy 4 stack arguments.) change the O32 syscall handler to always
load four arguments from the userspace stack even for syscalls that
require fewer or no arguments to be copied.  This removes a large table
from kernel space and need to maintain it.  It appeared that it was ok
the implementation chosen requires 16 bytes of readable stack space
above the user stack pointer.

Turned out a few threading implementations munmap the user stack before
the thread exits resulting in errors due to the unreadable stack.

We now treat any failed load as a if the loaded value was zero and let
the actual syscall deal with the situation.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

2 files changed, 52 insertions, 18 deletions

diff --git a/arch/mips/kernel/scall32-o32.S b/arch/mips/kernel/scall32-o32.S
index 6e8de80bb446..4cc13508d967 100644
--- a/arch/mips/kernel/scall32-o32.S
+++ b/arch/mips/kernel/scall32-o32.S
@@ -73,10 +73,11 @@ NESTED(handle_sys, PT_SIZE, sp)
         .set    noreorder
         .set    nomacro
 
-1:      user_lw(t5, 16(t0))             # argument #5 from usp
-4:      user_lw(t6, 20(t0))             # argument #6 from usp
-3:      user_lw(t7, 24(t0))             # argument #7 from usp
-2:      user_lw(t8, 28(t0))             # argument #8 from usp
+load_a4: user_lw(t5, 16(t0))            # argument #5 from usp
+load_a5: user_lw(t6, 20(t0))            # argument #6 from usp
+load_a6: user_lw(t7, 24(t0))            # argument #7 from usp
+load_a7: user_lw(t8, 28(t0))            # argument #8 from usp
+loads_done:
 
         sw      t5, 16(sp)              # argument #5 to ksp
         sw      t6, 20(sp)              # argument #6 to ksp
@@ -85,10 +86,10 @@ NESTED(handle_sys, PT_SIZE, sp)
         .set    pop
 
         .section __ex_table,"a"
-        PTR     1b,bad_stack
-        PTR     2b,bad_stack
-        PTR     3b,bad_stack
-        PTR     4b,bad_stack
+        PTR     load_a4, bad_stack_a4
+        PTR     load_a5, bad_stack_a5
+        PTR     load_a6, bad_stack_a6
+        PTR     load_a7, bad_stack_a7
         .previous
 
         lw      t0, TI_FLAGS($28)       # syscall tracing enabled?
@@ -153,8 +154,8 @@ syscall_trace_entry:
 /* ------------------------------------------------------------------------ */
 
         /*
-         * The stackpointer for a call with more than 4 arguments is bad.
-         * We probably should handle this case a bit more drastic.
+         * Our open-coded access area sanity test for the stack pointer
+         * failed. We probably should handle this case a bit more drastic.
          */
 bad_stack:
         li      v0, EFAULT
@@ -163,6 +164,22 @@ bad_stack:
         sw      t0, PT_R7(sp)
         j       o32_syscall_exit
 
+bad_stack_a4:
+        li      t5, 0
+        b       load_a5
+
+bad_stack_a5:
+        li      t6, 0
+        b       load_a6
+
+bad_stack_a6:
+        li      t7, 0
+        b       load_a7
+
+bad_stack_a7:
+        li      t8, 0
+        b       loads_done
+
         /*
          * The system call does not exist in this kernel
          */
diff --git a/arch/mips/kernel/scall64-o32.S b/arch/mips/kernel/scall64-o32.S
index d07b210fbeff..66d618bb2fa2 100644
--- a/arch/mips/kernel/scall64-o32.S
+++ b/arch/mips/kernel/scall64-o32.S
@@ -69,16 +69,17 @@ NESTED(handle_sys, PT_SIZE, sp)
         daddu   t1, t0, 32
         bltz    t1, bad_stack
 
-1:      lw      a4, 16(t0)              # argument #5 from usp
-2:      lw      a5, 20(t0)              # argument #6 from usp
-3:      lw      a6, 24(t0)              # argument #7 from usp
-4:      lw      a7, 28(t0)              # argument #8 from usp (for indirect syscalls)
+load_a4: lw     a4, 16(t0)              # argument #5 from usp
+load_a5: lw     a5, 20(t0)              # argument #6 from usp
+load_a6: lw     a6, 24(t0)              # argument #7 from usp
+load_a7: lw     a7, 28(t0)              # argument #8 from usp
+loads_done:
 
         .section __ex_table,"a"
-        PTR     1b, bad_stack
-        PTR     2b, bad_stack
-        PTR     3b, bad_stack
-        PTR     4b, bad_stack
+        PTR     load_a4, bad_stack_a4
+        PTR     load_a5, bad_stack_a5
+        PTR     load_a6, bad_stack_a6
+        PTR     load_a7, bad_stack_a7
         .previous
 
         li      t1, _TIF_WORK_SYSCALL_ENTRY
@@ -167,6 +168,22 @@ bad_stack:
         sd      t0, PT_R7(sp)
         j       o32_syscall_exit
 
+bad_stack_a4:
+        li      a4, 0
+        b       load_a5
+
+bad_stack_a5:
+        li      a5, 0
+        b       load_a6
+
+bad_stack_a6:
+        li      a6, 0
+        b       load_a7
+
+bad_stack_a7:
+        li      a7, 0
+        b       loads_done
+
 not_o32_scall:
         /*
          * This is not an o32 compatibility syscall, pass it on