Require at least libseccomp 2.5.5

Closes #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] https://github.com/NixOS/nixpkgs/pull/306070

(cherry picked from commit 73918b0ae4)

configure.ac

@@ -252,6 +252,17 @@ case "$host_os" in
                         [CXXFLAGS="$LIBSECCOMP_CFLAGS $CXXFLAGS"])
       have_seccomp=1
       AC_DEFINE([HAVE_SECCOMP], [1], [Whether seccomp is available and should be used for sandboxing.])
+      AC_COMPILE_IFELSE([
+        AC_LANG_SOURCE([[
+          #include <seccomp.h>
+          #ifndef __SNR_fchmodat2
+          # error "Missing support for fchmodat2"
+          #endif
+        ]])
+      ], [], [
+        echo "libseccomp is missing __SNR_fchmodat2. Please provide libseccomp 2.5.5 or later"
+        exit 1
+      ])
     else
       have_seccomp=
     fi

flake.nix

@@ -173,7 +173,13 @@
             boost
             lowdown-nix
           ]
-          ++ lib.optionals stdenv.isLinux [libseccomp]
+          ++ lib.optionals stdenv.isLinux [(libseccomp.overrideAttrs (_: rec {
+            version = "2.5.5";
+            src = fetchurl {
+              url = "https://github.com/seccomp/libseccomp/releases/download/v${version}/libseccomp-${version}.tar.gz";
+              hash = "sha256-JIosik2bmFiqa69ScSw0r+/PnJ6Ut23OAsHJqiX7M3U=";
+            };
+          }))]
           ++ lib.optional (stdenv.isLinux || stdenv.isDarwin) libsodium
           ++ lib.optional stdenv.hostPlatform.isx86_64 libcpuid;