The label .Llast_fixup@ is jumped to on page fault within the final byte set loop of memset (on < MIPSR6 architectures). For some reason, in this fault handler, the v1 register is randomly set to a2 & STORMASK. This clobbers v1 for the calling function. This can be observed with the following test code:
static int __init __attribute__((optimize("O0"))) test_clear_user(void) { register int t asm("v1"); char *test; int j, k;
pr_info("\n\n\nTesting clear_user\n"); test = vmalloc(PAGE_SIZE);
for (j = 256; j < 512; j++) { t = 0xa5a5a5a5; if ((k = clear_user(test + PAGE_SIZE - 256, j)) != j - 256) { pr_err("clear_user (%px %d) returned %d\n", test + PAGE_SIZE - 256, j, k); } if (t != 0xa5a5a5a5) { pr_err("v1 was clobbered to 0x%x!\n", t); } }
return 0; } late_initcall(test_clear_user);
Which demonstrates that v1 is indeed clobbered (MIPS64):
Testing clear_user v1 was clobbered to 0x1! v1 was clobbered to 0x2! v1 was clobbered to 0x3! v1 was clobbered to 0x4! v1 was clobbered to 0x5! v1 was clobbered to 0x6! v1 was clobbered to 0x7!
Since the number of bytes that could not be set is already contained in a2, the andi placing a value in v1 is not necessary and actively harmful in clobbering v1.
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: stable@vger.kernel.org Reported-by: James Hogan jhogan@kernel.org Signed-off-by: Matt Redfearn matt.redfearn@mips.com ---
arch/mips/lib/memset.S | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/mips/lib/memset.S b/arch/mips/lib/memset.S index fa3bec269331..84e91f4fdf53 100644 --- a/arch/mips/lib/memset.S +++ b/arch/mips/lib/memset.S @@ -258,7 +258,7 @@
.Llast_fixup@: jr ra - andi v1, a2, STORMASK + nop
.Lsmall_fixup@: PTR_SUBU a2, t1, a0