[PATCH] fortify: Ensure that __glibc_fortify condition is a constant [BZ #29141]

[PATCH] fortify: Ensure that __glibc_fortify condition is a constant [BZ #29141]

[Siddhesh Poyarekar]

The fix c8ee1c85 introduced a -1 check for object size without also
checking that object size is a constant.  Because of this, the tree
optimizer passes in gcc fail to fold away one of the branches in
__glibc_fortify and trips on a spurious Wstringop-overflow.  The warning
itself is incorrect and the branch does go away eventually in DCE in the
rtl passes in gcc, but the constant check is a helpful hint to simplify
code early, so add it in.

Resolves: BZ #29141
Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
---
 misc/sys/cdefs.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
index f1faf8292c..1c2b044a0d 100644
--- a/misc/sys/cdefs.h
+++ b/misc/sys/cdefs.h
@@ -164,7 +164,7 @@
 /* Length is known to be safe at compile time if the __L * __S <= __OBJSZ
    condition can be folded to a constant and if it is true, or unknown (-1) */
 #define __glibc_safe_or_unknown_len(__l, __s, __osz) \
-  ((__osz) == (__SIZE_TYPE__) -1					      \
+  ((__builtin_constant_p (__osz) && (__osz) == (__SIZE_TYPE__) -1)	      \
    || (__glibc_unsigned_or_positive (__l)				      \
        && __builtin_constant_p (__glibc_safe_len_cond ((__SIZE_TYPE__) (__l), \
 						       (__s), (__osz)))	      \
-- 
2.35.1

Re: [PATCH] fortify: Ensure that __glibc_fortify condition is a constant [BZ #29141]

[Florian Weimer]

* Siddhesh Poyarekar:

> The fix c8ee1c85 introduced a -1 check for object size without also
> checking that object size is a constant.  Because of this, the tree
> optimizer passes in gcc fail to fold away one of the branches in
> __glibc_fortify and trips on a spurious Wstringop-overflow.  The warning
> itself is incorrect and the branch does go away eventually in DCE in the
> rtl passes in gcc, but the constant check is a helpful hint to simplify
> code early, so add it in.
>
> Resolves: BZ #29141
> Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
> ---
>  misc/sys/cdefs.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
> index f1faf8292c..1c2b044a0d 100644
> --- a/misc/sys/cdefs.h
> +++ b/misc/sys/cdefs.h
> @@ -164,7 +164,7 @@
>  /* Length is known to be safe at compile time if the __L * __S <= __OBJSZ
>     condition can be folded to a constant and if it is true, or unknown (-1) */
>  #define __glibc_safe_or_unknown_len(__l, __s, __osz) \
> -  ((__osz) == (__SIZE_TYPE__) -1					      \
> +  ((__builtin_constant_p (__osz) && (__osz) == (__SIZE_TYPE__) -1)	      \
>     || (__glibc_unsigned_or_positive (__l)				      \
>         && __builtin_constant_p (__glibc_safe_len_cond ((__SIZE_TYPE__) (__l), \
>  						       (__s), (__osz)))	      \

I *think* this is okay based on localized review.

Thanks,
Florian