From a643f60c53876be0d57b4b7373770e6cb356fd13 Mon Sep 17 00:00:00 2001 From: Siddhesh Poyarekar Date: Wed, 20 Oct 2021 18:12:41 +0530 Subject: Make sure that the fortified function conditionals are constant In _FORTIFY_SOURCE=3, the size expression may be non-constant, resulting in branches in the inline functions remaining intact and causing a tiny overhead. Clang (and in future, gcc) make sure that the -1 case is always safe, i.e. any comparison of the generated expression with (size_t)-1 is always false so that bit is taken care of. The rest is avoidable since we want the _chk variant whenever we have a size expression and it's not -1. Rework the conditionals in a uniform way to clearly indicate two conditions at compile time: - Either the size is unknown (-1) or we know at compile time that the operation length is less than the object size. We can call the original function in this case. It could be that either the length, object size or both are non-constant, but the compiler, through range analysis, is able to fold the *comparison* to a constant. - The size and length are known and the compiler can see at compile time that operation length > object size. This is valid grounds for a warning at compile time, followed by emitting the _chk variant. For everything else, emit the _chk variant. This simplifies most of the fortified function implementations and at the same time, ensures that only one call from _chk or the regular function is emitted. Signed-off-by: Siddhesh Poyarekar Reviewed-by: Adhemerval Zanella --- misc/sys/cdefs.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 47 insertions(+) (limited to 'misc/sys/cdefs.h') diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h index f6ab95ab81..c353d2ec4d 100644 --- a/misc/sys/cdefs.h +++ b/misc/sys/cdefs.h @@ -150,6 +150,53 @@ # define __glibc_objsize(__o) __bos (__o) #endif +/* Compile time conditions to choose between the regular, _chk and _chk_warn + variants. These conditions should get evaluated to constant and optimized + away. */ + +#define __glibc_safe_len_cond(__l, __s, __osz) ((__l) <= (__osz) / (__s)) +#define __glibc_unsigned_or_positive(__l) \ + ((__typeof (__l)) 0 < (__typeof (__l)) -1 \ + || (__builtin_constant_p (__l) && (__l) > 0)) + +/* 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. The -1 check is + redundant because since it implies that __glibc_safe_len_cond is true. */ +#define __glibc_safe_or_unknown_len(__l, __s, __osz) \ + (__glibc_unsigned_or_positive (__l) \ + && __builtin_constant_p (__glibc_safe_len_cond ((__SIZE_TYPE__) (__l), \ + __s, __osz)) \ + && __glibc_safe_len_cond ((__SIZE_TYPE__) (__l), __s, __osz)) + +/* Conversely, we know at compile time that the length is safe if the + __L * __S <= __OBJSZ condition can be folded to a constant and if it is + false. */ +#define __glibc_unsafe_len(__l, __s, __osz) \ + (__glibc_unsigned_or_positive (__l) \ + && __builtin_constant_p (__glibc_safe_len_cond ((__SIZE_TYPE__) (__l), \ + __s, __osz)) \ + && !__glibc_safe_len_cond ((__SIZE_TYPE__) (__l), __s, __osz)) + +/* Fortify function f. __f_alias, __f_chk and __f_chk_warn must be + declared. */ + +#define __glibc_fortify(f, __l, __s, __osz, ...) \ + (__glibc_safe_or_unknown_len (__l, __s, __osz) \ + ? __ ## f ## _alias (__VA_ARGS__) \ + : (__glibc_unsafe_len (__l, __s, __osz) \ + ? __ ## f ## _chk_warn (__VA_ARGS__, __osz) \ + : __ ## f ## _chk (__VA_ARGS__, __osz))) \ + +/* Fortify function f, where object size argument passed to f is the number of + elements and not total size. */ + +#define __glibc_fortify_n(f, __l, __s, __osz, ...) \ + (__glibc_safe_or_unknown_len (__l, __s, __osz) \ + ? __ ## f ## _alias (__VA_ARGS__) \ + : (__glibc_unsafe_len (__l, __s, __osz) \ + ? __ ## f ## _chk_warn (__VA_ARGS__, (__osz) / (__s)) \ + : __ ## f ## _chk (__VA_ARGS__, (__osz) / (__s)))) \ + #if __GNUC_PREREQ (4,3) # define __warnattr(msg) __attribute__((__warning__ (msg))) # define __errordecl(name, msg) \ -- cgit v1.2.3