[PATCH] arm64: fix strnlen_user when count <= strlen

Hi Kyle,

On Thu, Jan 16, 2014 at 11:48:17PM +0000, Kyle McMartin wrote:

I received a bug report about the ruby test-suite failing on AArch64 when
attempting to pass MAX_ARG_STRLEN sized args to execv[1]. It was
expecting an E2BIG returned, but instead was receiving ENOMEM, and
concatenating the argument strings in funky ways.

The problem appeared to be in __strnlen_user on arm64, as when
instrumenting fs/exec.c to compare the results of the asm-generic
strnlen_user, I noticed an off-by-one on the result:
long-param-test: optimized strnlen_user (131072) and naive (131073) disagree!

As a result, fix strnlen_user to match expected behaviour as documented
in lib/strnlen_user.c, and return count+1 when count would be exceeded.

I didn't feel comfortable prodding the assembler, so I just worked
around it in the wrapper.

Actually, I have removed strnlen_user for 3.14. Could you try your test case
with our for-next branch please?

  git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-next/core

As for the issue you spotted, we probably need a fix for that to go into
stable kernels. Does the following (smaller patch) work for you?

Will

--->8

diff --git a/arch/arm64/lib/strnlen_user.S b/arch/arm64/lib/strnlen_user.S
index 7f7b176a5646..73f3335a2a45 100644
--- a/arch/arm64/lib/strnlen_user.S
+++ b/arch/arm64/lib/strnlen_user.S

@@ -37,6 +37,7 @@ ENTRY(__strnlen_user)
 USER(9f, ldrb	w3, [x0], #1	)
 	cbnz	w3, 1b
 2:	sub	x0, x0, x2
+	cinc	x0, x0, mi
 	ret
 ENDPROC(__strnlen_user)