linux-kernel - [PATCH] fs: use debug-only asserts around fd allocation and install

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Message-ID: <20250312161941.1261615-1-mjguzik@gmail.com>
Date: Wed, 12 Mar 2025 17:19:41 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: viro@...iv.linux.org.uk,
	jack@...e.cz,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH] fs: use debug-only asserts around fd allocation and install

This also restores the check which got removed in 52732bb9abc9ee5b
("fs/file.c: remove sanity_check and add likely/unlikely in alloc_fd()")
for performance reasons -- they no longer apply with a debug-only
variant.

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---

I have about 0 opinion whether this should be BUG or WARN, the code was
already inconsistent on this front. If you want the latter, I'll have 0
complaints if you just sed it and commit as yours.

This reminded me to sort out that litmus test for smp_rmb, hopefully
soon(tm) as it is now nagging me.

 fs/file.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 6c159ede55f1..09460ec74ef8 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -582,6 +582,7 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
 
 	__set_open_fd(fd, fdt, flags & O_CLOEXEC);
 	error = fd;
+	VFS_BUG_ON(rcu_access_pointer(fdt->fd[fd]) != NULL);
 
 out:
 	spin_unlock(&files->file_lock);
@@ -647,7 +648,7 @@ void fd_install(unsigned int fd, struct file *file)
 		rcu_read_unlock_sched();
 		spin_lock(&files->file_lock);
 		fdt = files_fdtable(files);
-		WARN_ON(fdt->fd[fd] != NULL);
+		VFS_BUG_ON(fdt->fd[fd] != NULL);
 		rcu_assign_pointer(fdt->fd[fd], file);
 		spin_unlock(&files->file_lock);
 		return;
@@ -655,7 +656,7 @@ void fd_install(unsigned int fd, struct file *file)
 	/* coupled with smp_wmb() in expand_fdtable() */
 	smp_rmb();
 	fdt = rcu_dereference_sched(files->fdt);
-	BUG_ON(fdt->fd[fd] != NULL);
+	VFS_BUG_ON(fdt->fd[fd] != NULL);
 	rcu_assign_pointer(fdt->fd[fd], file);
 	rcu_read_unlock_sched();
 }
-- 
2.43.0