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SubjectRe: [PATCH] mm: Move check for SHRINKER_NUMA_AWARE to do_shrink_slab()
Date
The reproducer can be reduced to:

#define _GNU_SOURCE
#include <endian.h>
#include <stdint.h>
#include <string.h>
#include <stdio.h>
#include <sys/syscall.h>
#include <sys/stat.h>
#include <sys/mount.h>
#include <unistd.h>
#include <fcntl.h>

const char path[] = "./file0";

int main()
{
mkdir(path, 0);
mount(path, path, "cgroup2", 0, 0);
chroot(path);
umount2(path, 0);
return 0;
}

and I've found two bugs (see attached patch). The issue is that
do_remount_sb() is called with fc == NULL from umount(), but both
cgroup_reconfigure() and do_remount_sb() dereference fc unconditionally.

But! I'm not sure why the reproducer works at all because the umount2() call
is *after* the chroot, so should fail on ENOENT before it even gets that far.
In fact, umount2() can be called multiple times, apparently successfully, and
doesn't actually unmount anything.

David
---
diff --git a/fs/super.c b/fs/super.c
index 3fe5d12b7697..321fbc244570 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -978,7 +978,10 @@ int do_remount_sb(struct super_block *sb, int sb_flags, void *data,
sb->s_op->remount_fs) {
if (sb->s_op->reconfigure) {
retval = sb->s_op->reconfigure(sb, fc);
- sb_flags = fc->sb_flags;
+ if (fc)
+ sb_flags = fc->sb_flags;
+ else
+ sb_flags = sb->s_flags;
if (retval == 0)
security_sb_reconfigure(fc);
} else {
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index f3238f38d152..48275fdce053 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1796,9 +1796,11 @@ static void apply_cgroup_root_flags(unsigned int root_flags)

static int cgroup_reconfigure(struct kernfs_root *kf_root, struct fs_context *fc)
{
- struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
+ if (fc) {
+ struct cgroup_fs_context *ctx = cgroup_fc2context(fc);

- apply_cgroup_root_flags(ctx->flags);
+ apply_cgroup_root_flags(ctx->flags);
+ }
return 0;
}

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