[PATCH] exec: fix off-by-one in binfmt max rewrite depth comment

[PATCH] exec: fix off-by-one in binfmt max rewrite depth comment

[Alan Urmancheev]

The loop in exec_binprm() permits depth values 0 through 5, up to 5
successive binfmt rewrites (setting bprm->interpreter) until the 6th
one would fail on depth > 5 and return -ELOOP. The comment claimed 4
levels, which was wrong. Adjusting the code to allow only 4 rewrites
would be breaking userland, so fix the comment and not the code.

Reproducer (a chain of shebanged scripts followed by an ELF binary):

    #!/bin/sh

    tmp=$(mktemp -d)
    echo $tmp
    cd $tmp

    mk () { echo $2 > $1; chmod +x $1; }

    for i in $(seq 4); do
    	mk $i "#!$((i + 1))"
    done

    mk 5 '#!/bin/true'
    ./1 &&
    echo '5 binfmt rewrites OK (1 -> 2 -> 3 -> 4 -> 5 -> /bin/true)'

    mk 5 '#!6'
    mk 6 '#!/bin/true'
    ./1 ||
    echo '6 binfmt rewrites KO (1 -> 2 -> 3 -> 4 -> 5 -> 6 -> /bin/true)'

Signed-off-by: Alan Urmancheev <alan.urman@gmail.com>
---
 fs/exec.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index b92fe7db1..d5993cedc 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1717,7 +1717,7 @@ static int exec_binprm(struct linux_binprm *bprm)
 	old_vpid = task_pid_nr_ns(current, task_active_pid_ns(current->parent));
 	rcu_read_unlock();
 
-	/* This allows 4 levels of binfmt rewrites before failing hard. */
+	/* This allows 5 levels of binfmt rewrites before failing hard. */
 	for (depth = 0;; depth++) {
 		struct file *exec;
 		if (depth > 5)
-- 
2.53.0

Re: [PATCH] exec: fix off-by-one in binfmt max rewrite depth comment

[Christian Brauner]

On Tue, 23 Jun 2026 01:23:22 -0400, Alan Urmancheev wrote:
> exec: fix off-by-one in binfmt max rewrite depth comment

Applied to the vfs.fixes branch of the vfs/vfs.git tree.
Patches in the vfs.fixes branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.fixes

[1/1] exec: fix off-by-one in binfmt max rewrite depth comment
      https://git.kernel.org/vfs/vfs/c/3146aada8527