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Fix PHP 8.5+ ZTS segfault on CentOS 7 by disabling IFUNC resolvers#20027

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Fix PHP 8.5+ ZTS segfault on CentOS 7 by disabling IFUNC resolvers#20027
realFlowControl wants to merge 1 commit intophp:PHP-8.5from
realFlowControl:florian/disable-ifunc-on-glibc-2.17

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@realFlowControl realFlowControl commented Oct 1, 2025 •
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CentOS 7's glibc (2.17) __builtin_cpu_init() causes IFUNC resolvers to crash during dynamic linking when detecting AVX2 support on a ZTS build on Amd64 hardware.

This PR disables IFUNC on Amd64 hardware when a glibc version <= 2.17 is detected.

The stacktrace for the segfault looks like this when executing a freshly build ./sapi/cli/php:

#0  0x0000555555972b5a in resolve_wchar_utf16be ()
#1  0x00007ffff7de6e6f in _dl_relocate_object () from /lib64/ld-linux-x86-64.so.2
#2  0x00007ffff7ddf99a in dl_main () from /lib64/ld-linux-x86-64.so.2
#3  0x00007ffff7df300e in _dl_sysdep_start () from /lib64/ld-linux-x86-64.so.2
#4  0x00007ffff7ddcbd1 in _dl_start () from /lib64/ld-linux-x86-64.so.2
#5  0x00007ffff7ddc148 in _start () from /lib64/ld-linux-x86-64.so.2
#6  0x0000000000000003 in ?? ()
#7  0x00007fffffffe287 in ?? ()
#8  0x00007fffffffe29a in ?? ()
#9  0x00007fffffffe29d in ?? ()
#10 0x0000000000000000 in ?? ()

This is even before PHP's main() function is called.

Arm64 is not affected because zend_cpu_supports_avx2(void) has an early return for Arm64:

php-src/Zend/zend_cpuinfo.h

Lines 200 to 210 in 52e1c9f

ZEND_NO_SANITIZE_ADDRESS
static inline int zend_cpu_supports_avx2(void) {
#ifdef __aarch64__
return 0;
#else
#ifdef PHP_HAVE_BUILTIN_CPU_INIT
__builtin_cpu_init();
#endif
return __builtin_cpu_supports("avx2");
#endif
}

Why NTS is not affected: I have no idea 🫤
Also: why this is not a problem on PHP 8.4 ZTS Amd64: no idea either

@realFlowControl
Fix PHP 8.5+ ZTS segfault on CentOS 7 by disabling IFUNC resolvers
0bf48f8 
CentOS 7's glibc lacks __builtin_cpu_init() support, causing IFUNC
resolvers to crash during dynamic linking when detecting AVX2 support.
Disable IFUNC and use runtime function pointer initialization instead.
@realFlowControl realFlowControl changed the base branch from master to PHP-8.5 October 1, 2025 12:13
@realFlowControl realFlowControl marked this pull request as ready for review October 1, 2025 12:23
@devnexen
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devnexen commented Oct 1, 2025 •
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Looking good, but I do not know if it is requested to be "consistent" with FreeBSD check style, e.g. getconf GNU_LIBC_VERSION | awk '{print $2}' | cut -d. -f2. cc @iluuu1994

@remicollet
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remicollet commented Oct 2, 2025

CentOS 7 is 11 years old and EOL for >1 year
https://blog.centos.org/2023/04/end-dates-are-coming-for-centos-stream-8-and-centos-linux-7/

There are so much things outdated there, a lot of PHP dependencies are no longer satisfied

I don't see the benefit of maintaining for dead targets

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realFlowControl commented Oct 2, 2025 •
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I don't see the benefit of maintaining for dead targets

Me neither but it is an "easy" fix and there are still folks using Linux with outdated glibc versions so I though: why not.

An alternative could be to document the workaround we are using currently and that is just adding ax_cv_have_func_attribute_ifunc=no to the arguments for ./configure for PHP 8.5 ZTS on old glibc versions:

$ ./configure ax_cv_have_func_attribute_ifunc=no --enable-zts --other-flags

IDK where to document this, most likely just having this comment posted here might already be enough for folks to find it 😉

@devnexen
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devnexen commented Oct 2, 2025

I remember Girgias and I had been discussing to level up the minimum os versions for the next major release. It was centos 8, solaris 10 ... for the next 8.6 I believe.

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@realFlowControl @devnexen @remicollet