Merge bitcoin/bitcoin#24659: util: add linkat to syscall sandbox (AllowFileSystem)

9809db3 util: add linkat to syscall sandbox (AllowFileSystem) (fanquake)

Pull request description:

  Should fix #24536.

ACKs for top commit:
  MarcoFalke:
    cr ACK 9809db3
  Rspigler:
    Tested ACK (commit 9809db3) - this fixes bitcoin/bitcoin#24536

Tree-SHA512: 2642f7dfa806e166fb32639a29b509b2edc8b919516c1f12430fc96f9887952395e157d71ef99fbaef8f7bcce1920530c24ecbce605b8a374b05d586f1f22a24

Author: MarcoFalke

src/util/syscall_sandbox.cpp

@@ -592,6 +592,7 @@ class SeccompPolicyBuilder
         allowed_syscalls.insert(__NR_getcwd);          // get current working directory
         allowed_syscalls.insert(__NR_getdents);        // get directory entries
         allowed_syscalls.insert(__NR_getdents64);      // get directory entries
+        allowed_syscalls.insert(__NR_linkat);          // create relative to a directory file descriptor
         allowed_syscalls.insert(__NR_lstat);           // get file status
         allowed_syscalls.insert(__NR_mkdir);           // create a directory
         allowed_syscalls.insert(__NR_newfstatat);      // get file status