libbpf: call dup2() syscall directly

We've ran into issues with using dup2() API in production setting, where
libbpf is linked into large production environment and ends up calling
unintended custom implementations of dup2(). These custom implementations
don't provide atomic FD replacement guarantees of dup2() syscall,
leading to subtle and hard to debug issues.

To prevent this in the future and guarantee that no libc implementation
will do their own custom non-atomic dup2() implementation, call dup2()
syscall directly with syscall(SYS_dup2).

Note that some architectures don't seem to provide dup2 and have dup3
instead. Try to detect and pick best syscall.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240119210201.1295511-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index 27e4e320e1a694736bdb5376a7b203242a71d240..58c547d473e007b3529f780ba657c3a609be4983 100644 (file)
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -15,6 +15,7 @@
 #include <linux/err.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <sys/syscall.h>
 #include <libelf.h>
 #include "relo_core.h"
 
@@ -555,6 +556,15 @@ static inline int ensure_good_fd(int fd)
        return fd;
 }
 
+static inline int sys_dup2(int oldfd, int newfd)
+{
+#ifdef __NR_dup2
+       return syscall(__NR_dup2, oldfd, newfd);
+#else
+       return syscall(__NR_dup3, oldfd, newfd, 0);
+#endif
+}
+
 /* Point *fixed_fd* to the same file that *tmp_fd* points to.
  * Regardless of success, *tmp_fd* is closed.
  * Whatever *fixed_fd* pointed to is closed silently.
@@ -563,7 +573,7 @@ static inline int reuse_fd(int fixed_fd, int tmp_fd)
 {
        int err;
 
-       err = dup2(tmp_fd, fixed_fd);
+       err = sys_dup2(tmp_fd, fixed_fd);
        err = err < 0 ? -errno : 0;
        close(tmp_fd); /* clean up temporary FD */
        return err;