libbpf: Fix the case of running as non-root with capabilities

When running rootless with special capabilities like:
FOWNER / DAC_OVERRIDE / DAC_READ_SEARCH

The "access" API will not make the proper check if there is really
access to a file or not.

>From the access man page:
"
The check is done using the calling process's real UID and GID, rather
than the effective IDs as is done when actually attempting an operation
(e.g., open(2)) on the file. Similarly, for the root user, the check
uses the set of permitted capabilities rather than the set of effective
capabilities; ***and for non-root users, the check uses an empty set of
capabilities.***
"

What that means is that for non-root user the access API will not do the
proper validation if the process really has permission to a file or not.

To resolve this this patch replaces all the access API calls with
faccessat with AT_EACCESS flag.

Signed-off-by: Jon Doron <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/btf.c

···
 	for (i = 0; i < ARRAY_SIZE(locations); i++) {
 		snprintf(path, PATH_MAX, locations[i], buf.release);
 
-		if (access(path, R_OK))
+		if (faccessat(AT_FDCWD, path, R_OK, AT_EACCESS))
 			continue;
 
 		btf = btf__parse(path, NULL);

tools/lib/bpf/libbpf.c

···
 	__u32 major, minor, patch;
 	struct utsname info;
 
-	if (access(ubuntu_kver_file, R_OK) == 0) {
+	if (faccessat(AT_FDCWD, ubuntu_kver_file, R_OK, AT_EACCESS) == 0) {
 		FILE *f;
 
 		f = fopen(ubuntu_kver_file, "r");
···
 	static int has_debugfs = -1;
 
 	if (has_debugfs < 0)
-		has_debugfs = access(DEBUGFS, F_OK) == 0;
+		has_debugfs = faccessat(AT_FDCWD, DEBUGFS, F_OK, AT_EACCESS) == 0;
 
 	return has_debugfs == 1;
 }
···
 				continue;
 			snprintf(result, result_sz, "%.*s/%s", seg_len, s, file);
 			/* ensure it has required permissions */
-			if (access(result, perm) < 0)
+			if (faccessat(AT_FDCWD, result, perm, AT_EACCESS) < 0)
 				continue;
 			pr_debug("resolved '%s' to '%s'\n", file, result);
 			return 0;

tools/lib/bpf/usdt.c

···
 	 * If this is not supported, USDTs with semaphores will not be supported.
 	 * Added in: a6ca88b241d5 ("trace_uprobe: support reference counter in fd-based uprobe")
 	 */
-	man->has_sema_refcnt = access(ref_ctr_sysfs_path, F_OK) == 0;
+	man->has_sema_refcnt = faccessat(AT_FDCWD, ref_ctr_sysfs_path, F_OK, AT_EACCESS) == 0;
 
 	return man;
 }