linux-kernel - Re: [syzbot] [overlayfs?] WARNING in ovl_encode_real

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Message-ID: <tencent_08A4E8A2ED86CE7C793E6CC02FBD6FF0960A@qq.com>
Date: Wed,  6 Nov 2024 10:43:28 +0800
From: Edward Adam Davis <eadavis@...com>
To: amir73il@...il.com
Cc: eadavis@...com,
	linux-kernel@...r.kernel.org,
	linux-unionfs@...r.kernel.org,
	miklos@...redi.hu,
	syzbot+ec07f6f5ce62b858579f@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [overlayfs?] WARNING in ovl_encode_real_fh

On Mon, 4 Nov 2024 20:30:41 +0100, Amir Goldstein <amir73il@...il.com> wrote:
> > When the memory is insufficient, the allocation of fh fails, which causes
> > the failure to obtain the dentry fid, and finally causes the dentry encoding
> > to fail.
> > Retry is used to avoid the failure of fh allocation caused by temporary
> > insufficient memory.
> >
> > #syz test
> >
> > diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> > index 2ed6ad641a20..1e027a3cf084 100644
> > --- a/fs/overlayfs/copy_up.c
> > +++ b/fs/overlayfs/copy_up.c
> > @@ -423,15 +423,22 @@ struct ovl_fh *ovl_encode_real_fh(struct ovl_fs *ofs, struct dentry *real,
> >         int fh_type, dwords;
> >         int buflen = MAX_HANDLE_SZ;
> >         uuid_t *uuid = &real->d_sb->s_uuid;
> > -       int err;
> > +       int err, rtt = 0;
> >
> >         /* Make sure the real fid stays 32bit aligned */
> >         BUILD_BUG_ON(OVL_FH_FID_OFFSET % 4);
> >         BUILD_BUG_ON(MAX_HANDLE_SZ + OVL_FH_FID_OFFSET > 255);
> >
> > +retry:
> >         fh = kzalloc(buflen + OVL_FH_FID_OFFSET, GFP_KERNEL);
> > -       if (!fh)
> > +       if (!fh) {
> > +               if (!rtt) {
> > +                       cond_resched();
> > +                       rtt++;
> > +                       goto retry;
> > +               }
> >                 return ERR_PTR(-ENOMEM);
> > +       }
> >
> >         /*
> >          * We encode a non-connectable file handle for non-dir, because we
> >
> 
> This endless loop is out of the question and anyway, syzbot reported
> a WARN_ON in line 448:
>             WARN_ON(fh_type == FILEID_INVALID))
> 
> How does that have to do with memory allocation failure?
> What am I missing?
Look following log, it in https://syzkaller.appspot.com/text?tag=CrashLog&x=178bf640580000:
[   64.050342][ T5103] FAULT_INJECTION: forcing a failure.
[   64.050342][ T5103] name failslab, interval 1, probability 0, space 0, times 0
[   64.055933][ T5103] CPU: 0 UID: 0 PID: 5103 Comm: syz-executor195 Not tainted 6.12.0-rc4-syzkaller-00047-gc2ee9f594da8 #0
[   64.060023][ T5103] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
[   64.063941][ T5103] Call Trace:
[   64.065199][ T5103]  <TASK>
[   64.066296][ T5103]  dump_stack_lvl+0x241/0x360
[   64.068028][ T5103]  ? __pfx_dump_stack_lvl+0x10/0x10
[   64.069939][ T5103]  ? __pfx__printk+0x10/0x10
[   64.071667][ T5103]  ? __kmalloc_cache_noprof+0x44/0x2c0
[   64.073756][ T5103]  ? __pfx___might_resched+0x10/0x10
[   64.075720][ T5103]  should_fail_ex+0x3b0/0x4e0
[   64.077525][ T5103]  should_failslab+0xac/0x100
[   64.079341][ T5103]  ? ovl_encode_real_fh+0xdf/0x410
[   64.081295][ T5103]  __kmalloc_cache_noprof+0x6c/0x2c0
[   64.083282][ T5103]  ? dput+0x37/0x2b0
[   64.084758][ T5103]  ovl_encode_real_fh+0xdf/0x410
[   64.086578][ T5103]  ? __pfx_ovl_encode_real_fh+0x10/0x10
[   64.088687][ T5103]  ? _raw_spin_unlock+0x28/0x50
[   64.090550][ T5103]  ovl_encode_fh+0x388/0xc20
[   64.092281][ T5103]  exportfs_encode_fh+0x1bd/0x3e0
[   64.094122][ T5103]  ovl_encode_real_fh+0x129/0x410
[   64.095883][ T5103]  ? __pfx_ovl_encode_real_fh+0x10/0x10
[   64.097852][ T5103]  ? bpf_lsm_capable+0x9/0x10
[   64.099620][ T5103]  ? capable+0x89/0xe0
[   64.101064][ T5103]  ovl_copy_up_flags+0x1068/0x46f0
> 
> Probably this WARN_ON as well as the one in line 446 should be
> relaxed because it is perfectly possible for fs to return negative or
> FILEID_INVALID for encoding a file handle even if fs supports encoding
> file handles.
> 

BR,
Edward