access(2) remains commonly used, for example on exec:
access("/etc/ld.so.preload", R_OK)
or when running gcc: strace -c gcc empty.c
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
0.00 0.000000 0 42 26 access
It falls down to do_faccessat without the AT_EACCESS flag, which in turn
results in allocation of new creds in order to modify fsuid/fsgid and
caps. This is a very expensive process single-threaded and most notably
multi-threaded, with numerous structures getting refed and unrefed on
imminent new cred destruction.
Turns out for typical consumers the resulting creds would be identical
and this can be checked upfront, avoiding the hard work.
An access benchmark plugged into will-it-scale running on Cascade Lake
shows:
test proc before after
access1 1 1310582 2908735 (+121%) # distinct files
access1 24 4716491 63822173 (+1353%) # distinct files
access2 24 2378041 5370335 (+125%) # same file
The above benchmarks are not integrated into will-it-scale, but can be
found in a pull request:
https://github.com/antonblanchard/will-it-scale/pull/36/files
Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
---
fs/open.c | 32 +++++++++++++++++++++++++++++++-
1 file changed, 31 insertions(+), 1 deletion(-)
diff --git a/fs/open.c b/fs/open.c
index 82c1a28b3308..c5bfc4e3df94 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -367,7 +367,37 @@ COMPAT_SYSCALL_DEFINE6(fallocate, int, fd, int, mode, compat_arg_u64_dual(offset
* access() needs to use the real uid/gid, not the effective uid/gid.
* We do this by temporarily clearing all FS-related capabilities and
* switching the fsuid/fsgid around to the real ones.
+ *
+ * Creating new credentials is expensive, so we try to skip doing it,
+ * which we can if the result would match what we already got.
*/
+static bool access_need_override_creds(int flags)
+{
+ const struct cred *cred;
+
+ if (flags & AT_EACCESS)
+ return false;
+
+ cred = current->cred;
+ if (!uid_eq(cred->fsuid, cred->uid) ||
+ !gid_eq(cred->fsgid, cred->gid))
+ return true;
+
+ if (!issecure(SECURE_NO_SETUID_FIXUP)) {
+ kuid_t root_uid = make_kuid(cred->user_ns, 0);
+ if (!uid_eq(cred->uid, root_uid)) {
+ if (!cap_isclear(cred->cap_effective))
+ return true;
+ } else {
+ if (!cap_isidentical(cred->cap_effective,
+ cred->cap_permitted))
+ return true;
+ }
+ }
+
+ return false;
+}
+
static const struct cred *access_override_creds(void)
{
const struct cred *old_cred;
@@ -436,7 +466,7 @@ static long do_faccessat(int dfd, const char __user *filename, int mode, int fla
if (flags & AT_EMPTY_PATH)
lookup_flags |= LOOKUP_EMPTY;
- if (!(flags & AT_EACCESS)) {
+ if (access_need_override_creds(flags)) {
old_cred = access_override_creds();
if (!old_cred)
return -ENOMEM;
--
2.34.1
