[CRIU] [PATCH] cr-check: Inspect errno of prctl calls

[Filipe Brandenburger]

When it fails, the prctl() syscall will return -1 and set `errno`
appropriately (instead of returning the error number code directly), so
we should check for ret < 0 and inspect errno instead of comparing the
return value with the error codes.

This bug was introduced in commit 8ceab588a5ce91 ("crtools: no more
linked with builtin syscall") which replaced sys_prctl() (which returned
the error number code directly) with prctl() (which returns -1 and sets
the `errno` variable.)

Also make all comparisons of "ret" explicit to check < 0 instead of the
implicit != 0 (PR_SET_MM is supposed to only return 0 or -1, but it's
good to be explicit in case this code is later refactored and that check
is not updated.)

Tested that now `criu check --ms` properly reports "prctl: PR_SET_MM is
not supported" (when indeed not supported) instead of the misleading
"prctl: One needs CAP_SYS_RESOURCE capability to perform testing."

Reported-by: Saied Kazemi <saied at google.com>
Cc: Laurent Dufour <ldufour at linux.vnet.ibm.com>
Signed-off-by: Filipe Brandenburger <filbranden at google.com>
---
 criu/cr-check.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/criu/cr-check.c b/criu/cr-check.c
index 2896b0280924..63e7197f5f65 100644
--- a/criu/cr-check.c
+++ b/criu/cr-check.c
@@ -177,7 +177,7 @@ static int check_prctl(void)
 	int ret;
 
 	ret = prctl(PR_GET_TID_ADDRESS, (unsigned long)&tid_addr, 0, 0, 0);
-	if (ret) {
+	if (ret < 0) {
 		pr_msg("prctl: PR_GET_TID_ADDRESS is not supported");
 		return -1;
 	}
@@ -186,7 +186,7 @@ static int check_prctl(void)
 	 * Either new or old interface must be supported in the kernel.
 	 */
 	ret = prctl(PR_SET_MM, PR_SET_MM_MAP_SIZE, (unsigned long)&size, 0, 0);
-	if (ret) {
+	if (ret < 0) {
 		if (!opts.check_ms_kernel) {
 			pr_msg("prctl: PR_SET_MM_MAP is not supported, which "
 			       "is required for restoring user namespaces\n");
@@ -195,8 +195,8 @@ static int check_prctl(void)
 			pr_warn("Skipping unssuported PR_SET_MM_MAP\n");
 
 		ret = prctl(PR_SET_MM, PR_SET_MM_BRK, brk(0), 0, 0);
-		if (ret) {
-			if (ret == -EPERM)
+		if (ret < 0) {
+			if (errno == -EPERM)
 				pr_msg("prctl: One needs CAP_SYS_RESOURCE capability to perform testing\n");
 			else
 				pr_msg("prctl: PR_SET_MM is not supported\n");
@@ -204,13 +204,13 @@ static int check_prctl(void)
 		}
 
 		ret = prctl(PR_SET_MM, PR_SET_MM_EXE_FILE, -1, 0, 0);
-		if (ret != -EBADF) {
+		if (ret < 0 && errno != -EBADF) {
 			pr_msg("prctl: PR_SET_MM_EXE_FILE is not supported (%d)\n", ret);
 			return -1;
 		}
 
 		ret = prctl(PR_SET_MM, PR_SET_MM_AUXV, (long)&user_auxv, sizeof(user_auxv), 0);
-		if (ret) {
+		if (ret < 0) {
 			pr_msg("prctl: PR_SET_MM_AUXV is not supported\n");
 			return -1;
 		}
-- 
2.7.0.rc3.207.g0ac5344