libseccomp: compilation broken by libseccomp v2.5.x

[jlsalvador]

systemd version the issue has been seen with

250

Used distribution

buildroot
bottlerocket-os (bottlerocket-os/bottlerocket#1815)

Linux kernel version used (uname -a)

5.15.12

CPU architecture issue was seen on

x86_64

Expected behaviour you didn't see

SCMP_SYS(openat2) doesn't returns 347 anymore. Instead returns undeclared __NR_openat2.

Unexpected behaviour you saw

libseccomp changed SCMP_SYS, so instead of returns 347 for openat2, now it returns __NR_openat2. You can include include/uapi/asm-generic/unistd.h in order to resolves __NR_openat2.
seccomp/libseccomp@bf16281
https://github.com/seccomp/libseccomp/blame/abad8a8f41fc13efbb95fc1ccaa3e181342bade7/include/seccomp-syscalls.h#L1304

Steps to reproduce the problem

Compiles with -Dseccomp=true and libseccomp 2.5.x (currently 2.5.3).

Additional program output to the terminal or log subsystem illustrating the issue

[411/982] Compiling C object src/shared/libsystemd-shared-250.a.p/seccomp-util.c.o
FAILED: src/shared/libsystemd-shared-250.a.p/seccomp-util.c.o 
[...]
../src/shared/seccomp-util.c: In function ‘seccomp_restrict_sxid’:
../src/shared/seccomp-util.c:2102:25: error: ‘__NR_openat2’ undeclared (first use in this function); did you mean ‘__openat_2’?
 2102 |                         SCMP_SYS(openat2),
      |                         ^~~~~~~~
../src/shared/seccomp-util.c:2102:25: note: each undeclared identifier is reported only once for each function it appears in
../src/shared/seccomp-util.c: In function ‘block_open_flag’:
../src/shared/seccomp-util.c:2245:25: error: ‘__NR_openat2’ undeclared (first use in this function); did you mean ‘__openat_2’?
 2245 |                         SCMP_SYS(openat2),
      |                         ^~~~~~~~

[jlsalvador]

Currently, there are two possible approach:

• Replace all SCMP_SYS() by the syscall numbers, or include just the libseccomp syscalls subset: (not recommended)
  SCMP_SYS(openat2) = 347, etc. (PR seccomp-util: fix build failure #21970)

• Copy the current file include/uapi/asm-generic/unistd.h from the kernel as defined at src/shared/linux/README.
  #include "linux/unistd.h" in src/shared/seccomp-util.c (PR seccomp-util: fix undeclared __NR_ #21971)