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authorChristian Brauner <brauner@kernel.org>2023-07-24 18:00:49 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2023-07-24 20:16:36 +0300
commit20ea1e7d13c1b544fe67c4a8dc3943bb1ab33e6f (patch)
tree028a89f1670036f51716ceadf448b55da937cac2
parent9e0ee0c7545c7ec012a53878e7687e05b87abc75 (diff)
downloadlinux-20ea1e7d13c1b544fe67c4a8dc3943bb1ab33e6f.tar.xz
file: always lock position for FMODE_ATOMIC_POS
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-a4abfd7103f3@kernel.org/ 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>
-rw-r--r--fs/file.c6
1 files changed, 2 insertions, 4 deletions
diff --git a/fs/file.c b/fs/file.c
index 7893ea161d77..35c62b54c9d6 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1042,10 +1042,8 @@ unsigned long __fdget_pos(unsigned int fd)
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;
}