[PATCH v2] security: commoncap: clarify CAP_FS_SET comment in cap_task_fix_setuid()

[Adriano Cordova <adrianox@gmail.com>]

CAP_FS_SET is exactly the set of capabilities that an fsuid of 0
historically granted (see CAP_FS_MASK in include/linux/capability.h),
so cap_task_fix_setuid() drops that set from the effective set when
fsuid leaves 0.  Replace the stale FIXME with this explanation.

Signed-off-by: Adriano Cordova <adrianox@gmail.com>
---
v2: reword the comment to something more concise.

 security/commoncap.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/security/commoncap.c b/security/commoncap.c
index 3399535808fe..1ba2ea2651fc 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -1169,11 +1169,13 @@ int cap_task_fix_setuid(struct cred *new, const struct cred *old, int flags)
 		break;
 
 	case LSM_SETID_FS:
-		/* juggle the capabilities to follow FSUID changes, unless
-		 * otherwise suppressed
+		/* Juggle the capabilities to follow FSUID changes, unless
+		 * otherwise suppressed.
 		 *
-		 * FIXME - is fsuser used for all CAP_FS_MASK capabilities?
-		 *          if not, we might be a bit too harsh here.
+		 * CAP_FS_SET is exactly the set of capabilities that an
+		 * fsuid of 0 historically granted (see CAP_FS_MASK in
+		 * <linux/capability.h>), so we drop that set when fsuid
+		 * leaves 0.
 		 */
 		if (!issecure(SECURE_NO_SETUID_FIXUP)) {
 			kuid_t root_uid = make_kuid(old->user_ns, 0);
-- 
2.51.0