On 9/6/23, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Tue, 5 Sept 2023 at 13:41, Mateusz Guzik <mjguzik@xxxxxxxxx> wrote: >> >> @@ -312,7 +314,15 @@ int vfs_fstatat(int dfd, const char __user >> *filename, >> struct filename *name; >> >> name = getname_flags(filename, >> getname_statx_lookup_flags(statx_flags), NULL); >> - ret = vfs_statx(dfd, name, statx_flags, stat, STATX_BASIC_STATS); >> + /* >> + * Hack: ugliness below damage controls glibc which reimplemented >> fstat >> + * on top of newfstatat(fd, "", buf, AT_EMPTY_PATH). We still pay >> for >> + * kmalloc and user access, but elide lockref. >> + */ >> + if (name->name[0] == '\0' && flags == AT_EMPTY_PATH && dfd >= 0) >> + ret = vfs_fstat(dfd, stat); >> + else >> + ret = vfs_statx(dfd, name, statx_flags, stat, >> STATX_BASIC_STATS); >> putname(name); > > I actually think I might prefer the earlier hacky thing, because it > avoids the whole nasty pathname allocation thing (ie the __getname() > dance in getname_flags(), and the addition of the pathname to the > audit records etc). > > I suspect your "there are no real loads that combine AT_EMPTY_PATH > with a path" comment is true. > > So if we have this short-circuit of the code, let's go all hog on it, > and avoid not just the RCU lookup (with lockref etc), but the pathname > allocation too. > Ok, I'm buggering off the subject. -- Mateusz Guzik <mjguzik gmail.com>
