https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127949
Bug ID: 127949
Summary: -fhardened overrides an explicit
-fno-stack-clash-protection
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: mark at markesler dot com
Target Milestone: ---
Created attachment 65873
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=65873&action=edit
Fix: test OPTION_SET_P instead of the flag value
The manual says -fhardened "only enables a particular option if it wasn't
already specified anywhere on the command line." For -fstack-clash-protection
it does not: an explicit -fno-stack-clash-protection is ignored, in either
argument order, without a warning.
$ cat t.c
void g(volatile char *);
void f(void) { volatile char big[1 << 20]; g(big); }
$ gcc -O2 -fno-stack-clash-protection -S -o - t.c | grep -c 'orb.*(%rsp)'
0
$ gcc -O2 -fhardened -fno-stack-clash-protection -S -o - t.c | grep -c
'orb.*(%rsp)'
1
t.c includes no headers, so it is its own preprocessed source.
Expected: no stack probes, as without -fhardened.
Actual: the probe loop (subq $4096, %rsp; orb $0, (%rsp)) is emitted.
This affects every release with -fhardened (GCC 14, 15 and 16) as well as
trunk, on x86_64 (also -m32, C++, -flto) and aarch64. Distributions that add
-fhardened by default leave packages no way to turn stack clash protection off.
The cause is in process_options (toplev.cc), which tests the flag's value
rather than whether it was given:
if (!flag_stack_clash_protection
&& flag_stack_check == NO_STACK_CHECK)
flag_stack_clash_protection = 1;
PR 114606 fixed the same pattern for -fcf-protection.
Attached are two patches. The first tests OPTION_SET_P
(flag_stack_clash_protection) instead, with three tests. The second adds the
-Whardened warning that -fno-stack-protector and
-ftrivial-auto-var-init=uninitialized already give, with two tests. Both were
bootstrapped and regression-tested on x86_64-pc-linux-gnu against d871a07, with
no new failures.
$ gcc -v
Using built-in specs.
COLLECT_GCC=/data/gcc-trunk-d871a07/inst/bin/gcc
COLLECT_LTO_WRAPPER=/data/gcc-trunk-d871a07/inst/libexec/gcc/x86_64-pc-linux-gnu/17.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../src/configure --prefix=/data/gcc-trunk-d871a07/inst
--disable-bootstrap --enable-languages=c,c++ --disable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 17.0.0 20261015 (experimental) (GCC)
Thanks.
