gcc-14: map_shadow_stack incorrect syscall number

[anarthal]

System: Ubuntu 24.04 (as per the ubuntu:24.04 Docker image)
Compiler: g++-14 (as per apt install g++-14 in the aforementioned image)
uname -a: Linux 2b064e2a8785 6.5.0-41-generic #41~22.04.2-Ubuntu SMP PREEMPT_DYNAMIC Mon Jun 3 11:32:55 UTC 2 x86_64 x86_64 x86_64 GNU/Linux
glibc version: 2.39
Variant: release
Library version: Boost develop branch

I'm getting a warning about a redefinition of __NR_map_shadow_stack. It's not causing any issues, in part because I'm only indirectly including the file (via Asio's spawn.hpp). It looks like <boost/context/fiber.hpp> is defining __NR_map_shadow_stack to 451, but the actual syscall number seems to be 453. This only happens in C++23, where <memory> includes all the syscall numbers.

For me it's just an annoying warning, but it seems it may point to a real bug. I don't know enough to provide further diagnostics.

Please note that this doesn't happen in the Godbolt environment, which uses an older glibc version. The ubuntu 24.04 gcc-14 seems to have __CET__ defined by default.

Repro instructions:

$> cd $BOOST_ROOT
$> cat libs/mysql/repro/Jamfile
exe repro :
    repro.cpp
    /boost/context//boost_context ;

$> cat libs/mysql/repro/repro.cpp
#include <boost/context/fiber.hpp>

int main() {}

$> ./b2 toolset=gcc-14 cxxstd=23 libs/mysql/repro warnings-as-errors=on
Performing configuration checks

    - default address-model    : 64-bit (cached) [1]
    - default architecture     : x86 (cached) [1]
    - symlinks supported       : yes (cached)

[1] gcc-14
...patience...
...found 639 targets...
...updating 9 targets...
gcc.compile.c++ bin.v2/libs/mysql/repro/gcc-14/debug/x86_64/cxxstd-23-iso/threading-multi/visibility-hidden/repro.o
In file included from ./boost/context/fiber.hpp:12,
                 from libs/mysql/repro/repro.cpp:1:
./boost/context/fiber_fcontext.hpp:52:11: error: "__NR_map_shadow_stack" redefined [-Werror]
   52 | #  define __NR_map_shadow_stack 451
      |           ^~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/x86_64-linux-gnu/asm/unistd.h:20,
                 from /usr/include/x86_64-linux-gnu/sys/syscall.h:24,
                 from /usr/include/syscall.h:1,
                 from /usr/include/c++/14/bits/atomic_wait.h:47,
                 from /usr/include/c++/14/bits/atomic_base.h:42,
                 from /usr/include/c++/14/bits/shared_ptr_atomic.h:33,
                 from /usr/include/c++/14/memory:81,
                 from ./boost/context/fiber_fcontext.hpp:18:
/usr/include/x86_64-linux-gnu/asm/unistd_64.h:368:9: note: this is the location of the previous definition
  368 | #define __NR_map_shadow_stack 453
      |         ^~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors

    "g++"   -std=c++23 -fvisibility-inlines-hidden -fPIC -m64 -pthread -O0 -fno-inline -Wall -Werror -g -fvisibility=hidden  -DBOOST_ALL_NO_LIB=1 -DBOOST_CONTEXT_DYN_LINK=1   -I"."  -c -o "bin.v2/libs/mysql/repro/gcc-14/debug/x86_64/cxxstd-23-iso/threading-multi/visibility-hidden/repro.o" "libs/mysql/repro/repro.cpp"

...failed gcc.compile.c++ bin.v2/libs/mysql/repro/gcc-14/debug/x86_64/cxxstd-23-iso/threading-multi/visibility-hidden/repro.o...
...skipped <pbin.v2/libs/mysql/repro/gcc-14/debug/x86_64/cxxstd-23-iso/threading-multi/visibility-hidden>repro for lack of <pbin.v2/libs/mysql/repro/gcc-14/debug/x86_64/cxxstd-23-iso/threading-multi/visibility-hidden>repro.o...

...updated 7 targets...

...skipped 1 target...
   <pbin.v2/libs/mysql/repro/gcc-14/debug/x86_64/cxxstd-23-iso/threading-multi/visibility-hidden>repro

...failed updating 1 target...
   gcc.compile.c++ bin.v2/libs/mysql/repro/gcc-14/debug/x86_64/cxxstd-23-iso/threading-multi/visibility-hidden/repro.o

[olk]

Might be fixed by PR 259 (#259). Could you test it?

[arnaud-lb]

This is a separate issue.

The syscall number appears to have changed since the implementation of shadow stacks in boost/context.

The impact is that boost/context will fail to setup a shadow stack when SHSTK is enabled (which is unlikely currently), and will crash.

In https://github.com/php/php-src/pull/14027/files we define SYS_map_shadow_stack to 453, and only when it's not already defined.

[anarthal]

Confirming that this still happens, and having read the source, I agree with @arnaud-lb on the diagnostics. I don't know the specifics on gcc-14 and why I'm encountering this without setting any flags, but this looks like a problem.