Closes NixOS#10585

As it turns out, libseccomp maintains an internal syscall table and
validates each rule against it. This means that when using libseccomp
2.5.4 or older, one may pass `452` as syscall number against it, but
since it doesn't exist in the internal structure, `libseccomp` will refuse
to create a filter for that. This happens with nixpkgs-23.11, i.e. on
stable NixOS and when building Nix against the project's flake.

To work around that

* a backport of libseccomp 2.5.5 on upstream nixpkgs has been
  scheduled[1].

* the package now uses libseccomp 2.5.5 on its own already. This is to
  provide a quick fix since the correct fix for 23.11 is still a staging cycle
  away.

It must not be possible to build a Nix with an incompatible libseccomp
version (nothing can be built in a sandbox on Linux!), so configure.ac
rejects libseccomp if `__SNR_fchmodat2` is not defined.

We still need the compat header though since `SCMP_SYS(fchmodat2)`
internally transforms this into `__SNR_fchmodat2` which points to
`__NR_fchmodat2` from glibc 2.39, so it wouldn't build on glibc 2.38.
The updated syscall table from libseccomp 2.5.5 is NOT used for that
step, but used later, so we need both, our compat header and their
syscall table 🤷

[1] NixOS/nixpkgs#306070

Closes NixOS#10585

As it turns out, libseccomp maintains an internal syscall table and
validates each rule against it. This means that when using libseccomp
2.5.4 or older, one may pass `452` as syscall number against it, but
since it doesn't exist in the internal structure, `libseccomp` will refuse
to create a filter for that. This happens with nixpkgs-23.11, i.e. on
stable NixOS and when building Nix against the project's flake.

To work around that

* a backport of libseccomp 2.5.5 on upstream nixpkgs has been
  scheduled[1].

* the package now uses libseccomp 2.5.5 on its own already. This is to
  provide a quick fix since the correct fix for 23.11 is still a staging cycle
  away.

It must not be possible to build a Nix with an incompatible libseccomp
version (nothing can be built in a sandbox on Linux!), so configure.ac
rejects libseccomp if `__SNR_fchmodat2` is not defined.

We still need the compat header though since `SCMP_SYS(fchmodat2)`
internally transforms this into `__SNR_fchmodat2` which points to
`__NR_fchmodat2` from glibc 2.39, so it wouldn't build on glibc 2.38.
The updated syscall table from libseccomp 2.5.5 is NOT used for that
step, but used later, so we need both, our compat header and their
syscall table 🤷

[1] NixOS/nixpkgs#306070

(cherry picked from commit 73918b0)

With Linux kernel >=6.6 & glibc 2.39 a `fchmodat2(2)` is available that
isn't filtered away by the libseccomp sandbox.

Being able to use this to bypass that restriction has surprising results
for some builds such as lxc[1]:

> With kernel ≥6.6 and glibc 2.39, lxc's install phase uses fchmodat2,
> which slips through https://github.com/NixOS/nix/blob/9b88e5284608116b7db0dbd3d5dd7a33b90d52d7/src/libstore/build/local-derivation-goal.cc#L1650-L1663.
> The fixupPhase then uses fchmodat, which fails.
> With older kernel or glibc, setting the suid bit fails in the
> install phase, which is not treated as fatal, and then the
> fixup phase does not try to set it again.

Please note that there are still ways to bypass this sandbox[2] and this is
mostly a fix for the breaking builds.

This change works by creating a syscall filter for the `fchmodat2`
syscall (number 452 on most systems). The problem is that glibc 2.39
is needed to have the correct syscall number available via
`__NR_fchmodat2` / `__SNR_fchmodat2`, but this flake is still on
nixpkgs 23.11. To have this change everywhere and not dependent on the
glibc this package is built against, I added a header
"fchmodat2-compat.hh" that sets the syscall number based on the
architecture. On most platforms its 452 according to glibc with a few
exceptions:

    $ rg --pcre2 'define __NR_fchmodat2 (?!452)'
    sysdeps/unix/sysv/linux/x86_64/x32/arch-syscall.h
    58:#define __NR_fchmodat2 1073742276

    sysdeps/unix/sysv/linux/mips/mips64/n32/arch-syscall.h
    67:#define __NR_fchmodat2 6452

    sysdeps/unix/sysv/linux/mips/mips64/n64/arch-syscall.h
    62:#define __NR_fchmodat2 5452

    sysdeps/unix/sysv/linux/mips/mips32/arch-syscall.h
    70:#define __NR_fchmodat2 4452

    sysdeps/unix/sysv/linux/alpha/arch-syscall.h
    59:#define __NR_fchmodat2 562

I added a small regression-test to the setuid integration-test that
attempts to set the suid bit on a file using the fchmodat2 syscall.
I confirmed that the test fails without the change in
local-derivation-goal.

Additionally, we require libseccomp 2.5.5 or greater now: as it turns
out, libseccomp maintains an internal syscall table and
validates each rule against it. This means that when using libseccomp
2.5.4 or older, one may pass `452` as syscall number against it, but
since it doesn't exist in the internal structure, `libseccomp` will refuse
to create a filter for that. This happens with nixpkgs-23.11, i.e. on
stable NixOS and when building Lix against the project's flake.

To work around that

* a backport of libseccomp 2.5.5 on upstream nixpkgs has been
  scheduled[3].

* the package now uses libseccomp 2.5.5 on its own already. This is to
  provide a quick fix since the correct fix for 23.11 is still a staging cycle
  away.

We still need the compat header though since `SCMP_SYS(fchmodat2)`
internally transforms this into `__SNR_fchmodat2` which points to
`__NR_fchmodat2` from glibc 2.39, so it wouldn't build on glibc 2.38.
The updated syscall table from libseccomp 2.5.5 is NOT used for that
step, but used later, so we need both, our compat header and their
syscall table 🤷

Relevant PRs in CppNix:

* NixOS/nix#10591
* NixOS/nix#10501

[1] NixOS/nixpkgs#300635 (comment)
[2] NixOS/nixpkgs#300635 (comment)
[3] NixOS/nixpkgs#306070

(cherry picked from commit ba68045)
Change-Id: I6921ab5a363188c6bff617750d00bb517276b7fe

With Linux kernel >=6.6 & glibc 2.39 a `fchmodat2(2)` is available that
isn't filtered away by the libseccomp sandbox.

Being able to use this to bypass that restriction has surprising results
for some builds such as lxc[1]:

> With kernel ≥6.6 and glibc 2.39, lxc's install phase uses fchmodat2,
> which slips through https://github.com/NixOS/nix/blob/9b88e5284608116b7db0dbd3d5dd7a33b90d52d7/src/libstore/build/local-derivation-goal.cc#L1650-L1663.
> The fixupPhase then uses fchmodat, which fails.
> With older kernel or glibc, setting the suid bit fails in the
> install phase, which is not treated as fatal, and then the
> fixup phase does not try to set it again.

Please note that there are still ways to bypass this sandbox[2] and this is
mostly a fix for the breaking builds.

This change works by creating a syscall filter for the `fchmodat2`
syscall (number 452 on most systems). The problem is that glibc 2.39
is needed to have the correct syscall number available via
`__NR_fchmodat2` / `__SNR_fchmodat2`, but this flake is still on
nixpkgs 23.11. To have this change everywhere and not dependent on the
glibc this package is built against, I added a header
"fchmodat2-compat.hh" that sets the syscall number based on the
architecture. On most platforms its 452 according to glibc with a few
exceptions:

    $ rg --pcre2 'define __NR_fchmodat2 (?!452)'
    sysdeps/unix/sysv/linux/x86_64/x32/arch-syscall.h
    58:#define __NR_fchmodat2 1073742276

    sysdeps/unix/sysv/linux/mips/mips64/n32/arch-syscall.h
    67:#define __NR_fchmodat2 6452

    sysdeps/unix/sysv/linux/mips/mips64/n64/arch-syscall.h
    62:#define __NR_fchmodat2 5452

    sysdeps/unix/sysv/linux/mips/mips32/arch-syscall.h
    70:#define __NR_fchmodat2 4452

    sysdeps/unix/sysv/linux/alpha/arch-syscall.h
    59:#define __NR_fchmodat2 562

I added a small regression-test to the setuid integration-test that
attempts to set the suid bit on a file using the fchmodat2 syscall.
I confirmed that the test fails without the change in
local-derivation-goal.

Additionally, we require libseccomp 2.5.5 or greater now: as it turns
out, libseccomp maintains an internal syscall table and
validates each rule against it. This means that when using libseccomp
2.5.4 or older, one may pass `452` as syscall number against it, but
since it doesn't exist in the internal structure, `libseccomp` will refuse
to create a filter for that. This happens with nixpkgs-23.11, i.e. on
stable NixOS and when building Lix against the project's flake.

To work around that

* a backport of libseccomp 2.5.5 on upstream nixpkgs has been
  scheduled[3].

* the package now uses libseccomp 2.5.5 on its own already. This is to
  provide a quick fix since the correct fix for 23.11 is still a staging cycle
  away.

We still need the compat header though since `SCMP_SYS(fchmodat2)`
internally transforms this into `__SNR_fchmodat2` which points to
`__NR_fchmodat2` from glibc 2.39, so it wouldn't build on glibc 2.38.
The updated syscall table from libseccomp 2.5.5 is NOT used for that
step, but used later, so we need both, our compat header and their
syscall table 🤷

Relevant PRs in CppNix:

* NixOS/nix#10591
* NixOS/nix#10501

[1] NixOS/nixpkgs#300635 (comment)
[2] NixOS/nixpkgs#300635 (comment)
[3] NixOS/nixpkgs#306070

(cherry picked from commit ba68045)
Change-Id: I6921ab5a363188c6bff617750d00bb517276b7fe