From: Christian Brauner brauner@kernel.org
commit 20ea1e7d13c1b544fe67c4a8dc3943bb1ab33e6f upstream.
The pidfd_getfd() system call allows a caller with ptrace_may_access() abilities on another process to steal a file descriptor from this process. This system call is used by debuggers, container runtimes, system call supervisors, networking proxies etc. So while it is a special interest system call it is used in common tools.
That ability ends up breaking our long-time optimization in fdget_pos(), which "knew" that if we had exclusive access to the file descriptor nobody else could access it, and we didn't need the lock for the file position.
That check for file_count(file) was always fairly subtle - it depended on __fdget() not incrementing the file count for single-threaded processes and thus included that as part of the rule - but it did mean that we didn't need to take the lock in all those traditional unix process contexts.
So it's sad to see this go, and I'd love to have some way to re-instate the optimization. At the same time, the lock obviously isn't ever contended in the case we optimized, so all we were optimizing away is the atomics and the cacheline dirtying. Let's see if anybody even notices that the optimization is gone.
Link: https://lore.kernel.org/linux-fsdevel/20230724-vfs-fdget_pos-v1-1-a4abfd7103... Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall") Cc: stable@kernel.org Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/file.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-)
--- a/fs/file.c +++ b/fs/file.c @@ -1042,10 +1042,8 @@ unsigned long __fdget_pos(unsigned int f struct file *file = (struct file *)(v & ~3);
if (file && (file->f_mode & FMODE_ATOMIC_POS)) { - if (file_count(file) > 1) { - v |= FDPUT_POS_UNLOCK; - mutex_lock(&file->f_pos_lock); - } + v |= FDPUT_POS_UNLOCK; + mutex_lock(&file->f_pos_lock); } return v; }