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Subject[PATCH RFC] proc: Don't retain negative dentries
Date
The referenced commit 1da4d377f94 ("proc: revalidate misc dentries")
caused following userspace code to access a stale /proc/net/dev
after the network namespace was changed:

system("ip netns add testns");

printf("default:\n"); {
int devinfd = open("/proc/net/dev", O_RDONLY);
sendfile(STDOUT_FILENO, devinfd, NULL, 4096);
close(devinfd);
}

printf("testns:\n"); {
int ns_fd = open("/var/run/netns/testns", O_RDONLY);
setns(ns_fd, 0);

int devinfd = open("/proc/net/dev", O_RDONLY);
sendfile(STDOUT_FILENO, devinfd, NULL, 4096);
close(devinfd);

close(ns_fd);
}

Despite switching the network namespace, the read access from the
newly opened file gave back what the very first read in the default
network namespace returned.

This doesn't occur if /proc/net/dev is opened within a new process,
which might explain why this wasn't noticed previously.

As I don't see a reason why one would keep negative dentries for procfs
at all, amend the code not to do this anymore.

Fixes: 1da4d377f94 ("proc: Don't retain negative dentries")
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---

Notes:
Alexey, could you check this doesn't lead to a regression
concerning the bug you fixed?

fs/proc/generic.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index 8ae109429a88..412b3c52d5d5 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -231,7 +231,7 @@ static int proc_misc_d_revalidate(struct dentry *dentry, unsigned int flags)

static int proc_misc_d_delete(const struct dentry *dentry)
{
- return atomic_read(&PDE(d_inode(dentry))->in_use) < 0;
+ return 1; // Don't retain negative dentries
}

static const struct dentry_operations proc_misc_dentry_ops = {
--
2.19.0
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