[PATCH bpf-next v2] libbpf: Set MFD_NOEXEC_SEAL when creating memfd

[Daniel Xu]

Starting from 105ff5339f49 ("mm/memfd: add MFD_NOEXEC_SEAL and
MFD_EXEC") and until 1717449b4417 ("memfd: drop warning for missing
exec-related flags"), the kernel would print a warning if neither
MFD_NOEXEC_SEAL nor MFD_EXEC is set in memfd_create().

If libbpf runs on on a kernel between these two commits (eg. on an
improperly backported system), it'll trigger this warning.

To avoid this warning (and also be more secure), explicitly set
MFD_NOEXEC_SEAL. But since libbpf can be run on potentially very old
kernels, leave a fallback for kernels without MFD_NOEXEC_SEAL support.

Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
---
Changes since v1:
* Update commit message with pointers to recent memfd warning changes

 tools/lib/bpf/libbpf.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 66173ddb5a2d..46492cc0927d 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -1731,12 +1731,24 @@ static int sys_memfd_create(const char *name, unsigned flags)
 #ifndef MFD_CLOEXEC
 #define MFD_CLOEXEC 0x0001U
 #endif
+#ifndef MFD_NOEXEC_SEAL
+#define MFD_NOEXEC_SEAL 0x0008U
+#endif
 
 static int create_placeholder_fd(void)
 {
+	unsigned int flags = MFD_CLOEXEC | MFD_NOEXEC_SEAL;
+	const char *name = "libbpf-placeholder-fd";
 	int fd;
 
-	fd = ensure_good_fd(sys_memfd_create("libbpf-placeholder-fd", MFD_CLOEXEC));
+	fd = ensure_good_fd(sys_memfd_create(name, flags));
+	if (fd >= 0)
+		return fd;
+	else if (errno != EINVAL)
+		return -errno;
+
+	/* Possibly running on kernel without MFD_NOEXEC_SEAL */
+	fd = ensure_good_fd(sys_memfd_create(name, flags & ~MFD_NOEXEC_SEAL));
 	if (fd < 0)
 		return -errno;
 	return fd;
-- 
2.47.1