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Date: Mon, 30 Aug 2021 15:34:01 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Kees Cook <keescook@...omium.org>
Cc: linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
"Gustavo A. R. Silva" <gustavoars@...nel.org>,
Rasmus Villemoes <linux@...musvillemoes.dk>,
Keith Packard <keithp@...thp.com>,
Dan Williams <dan.j.williams@...el.com>,
Daniel Vetter <daniel.vetter@...ll.ch>,
clang-built-linux@...glegroups.com,
linux-hardening@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v3 0/5] Enable -Warray-bounds and -Wzero-length-bounds
On Mon, Aug 30, 2021 at 01:16:41PM -0700, Kees Cook wrote:
> On Mon, Aug 30, 2021 at 11:44:54AM -0700, Nathan Chancellor wrote:
> > arch/powerpc/kernel/signal_32.c:780:2: error: array index 3 is past the end of the array (which contains 1 element) [-Werror,-Warray-bounds]
> > unsafe_put_sigset_t(&frame->uc.uc_sigmask, oldset, failed);
> > ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Or is this a Clang DCE failure?
>
> #define unsafe_put_compat_sigset(compat, set, label) do { \
> compat_sigset_t __user *__c = compat; \
> const sigset_t *__s = set; \
> \
> switch (_NSIG_WORDS) { \
> case 4: \
> unsafe_put_user(__s->sig[3] >> 32, &__c->sig[7], label); \
> unsafe_put_user(__s->sig[3], &__c->sig[6], label); \
> fallthrough; \
> case 3: \
> unsafe_put_user(__s->sig[2] >> 32, &__c->sig[5], label); \
> unsafe_put_user(__s->sig[2], &__c->sig[4], label); \
> fallthrough; \
> case 2: \
> unsafe_put_user(__s->sig[1] >> 32, &__c->sig[3], label); \
> unsafe_put_user(__s->sig[1], &__c->sig[2], label); \
> fallthrough; \
> case 1: \
> unsafe_put_user(__s->sig[0] >> 32, &__c->sig[1], label); \
> unsafe_put_user(__s->sig[0], &__c->sig[0], label); \
> } \
> } while (0)
>
> if "set" has only 1 element, then _NSIG_WORDS must be 1. The warnings
> are coming from cases 4 and 3. (But why not 2, which would also access
> beyond the end?)
I trimmed the warnings down otherwise it would have been 400 lines long
:) it did warn for the 2 case.
Clang does not like the use of asm goto in unsafe_put_user on powerpc it
seems:
$ cat warray-bounds.c
#define NSIG_WORDS 1
typedef struct {
unsigned long sig[NSIG_WORDS];
} sigset_t;
int handle_rt_signal32_bad(sigset_t *);
int handle_rt_signal32_bad(sigset_t *oldset)
{
switch (NSIG_WORDS) {
case 4:
__asm__ goto("" : : "r"(oldset->sig[3] >> 32) : : failed);
__asm__ goto("" : : "r"(oldset->sig[3]) : : failed);
__attribute__((fallthrough));
case 3:
__asm__ goto("" : : "r"(oldset->sig[2] >> 32) : : failed);
__asm__ goto("" : : "r"(oldset->sig[2]) : : failed);
__attribute__((fallthrough));
case 2:
__asm__ goto("" : : "r"(oldset->sig[1] >> 32) : : failed);
__asm__ goto("" : : "r"(oldset->sig[1]) : : failed);
__attribute__((fallthrough));
case 1:
__asm__ goto("" : : "r"(oldset->sig[0] >> 32) : : failed);
__asm__ goto("" : : "r"(oldset->sig[0]) : : failed);
}
return 0;
failed:
return 1;
}
void normal_array_access(unsigned long);
int handle_rt_signal32_good(sigset_t *);
int handle_rt_signal32_good(sigset_t *oldset)
{
switch (NSIG_WORDS) {
case 4:
normal_array_access(oldset->sig[3] >> 32);
normal_array_access(oldset->sig[3]);
__attribute__((fallthrough));
case 3:
normal_array_access(oldset->sig[2] >> 32);
normal_array_access(oldset->sig[2]);
__attribute__((fallthrough));
case 2:
normal_array_access(oldset->sig[1] >> 32);
normal_array_access(oldset->sig[1]);
__attribute__((fallthrough));
case 1:
normal_array_access(oldset->sig[0] >> 32);
normal_array_access(oldset->sig[0]);
}
return 0;
}
$ clang -fsyntax-only -Weverything warray-bounds.c
warray-bounds.c:12:27: warning: array index 3 is past the end of the array (which contains 1 element) [-Warray-bounds]
__asm__ goto("" : : "r"(oldset->sig[3] >> 32) : : failed);
^ ~
warray-bounds.c:4:2: note: array 'sig' declared here
unsigned long sig[NSIG_WORDS];
^
warray-bounds.c:16:27: warning: array index 2 is past the end of the array (which contains 1 element) [-Warray-bounds]
__asm__ goto("" : : "r"(oldset->sig[2] >> 32) : : failed);
^ ~
warray-bounds.c:4:2: note: array 'sig' declared here
unsigned long sig[NSIG_WORDS];
^
warray-bounds.c:20:27: warning: array index 1 is past the end of the array (which contains 1 element) [-Warray-bounds]
__asm__ goto("" : : "r"(oldset->sig[1] >> 32) : : failed);
^ ~
warray-bounds.c:4:2: note: array 'sig' declared here
unsigned long sig[NSIG_WORDS];
^
3 warnings generated.
$ gcc -fsyntax-only -Wall -Wextra -Wpedantic warray-bounds.c
godbolt link: https://godbolt.org/z/8xYojs1WY
I've reported this on LLVM's bug tracker to see what the clang
developers can do with you on CC:
https://bugs.llvm.org/show_bug.cgi?id=51682
Cheers,
Nathan
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