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Date:   Thu, 18 Jun 2020 20:31:05 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Eric Sandeen <sandeen@...hat.com>
Cc:     "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 0/1] ext4: fix potential negative array index in do_split

On Jun 17, 2020, at 1:01 PM, Eric Sandeen <sandeen@...hat.com> wrote:
> 
> We recently had a report of a panic in do_split; the filesystem in question
> panicked a distribution kernel when trying to add a new directory entry;
> the behavior/bug persists upstream.
> 
> The directory block in question had lots of unused and un-coalesced
> entries, like this, printed from the loop in ext4_insert_dentry():
> 
> [32778.024654] reclen 44 for name len 36
> [32778.028745] start: de ffff9f4cb5309800 top ffff9f4cb5309bd4
> [32778.034971]  offset 0 nlen 28 rlen 40, rlen-nlen 12, reclen 44 name <empty>
> [32778.042744]  offset 40 nlen 28 rlen 28, rlen-nlen 0, reclen 44 name <empty>
> [32778.050521]  offset 68 nlen 32 rlen 32, rlen-nlen 0, reclen 44 name <empty>
> [32778.058294]  offset 100 nlen 28 rlen 28, rlen-nlen 0, reclen 44 name <empty>
> [32778.066166]  offset 128 nlen 28 rlen 28, rlen-nlen 0, reclen 44 name <empty>
> [32778.074035]  offset 156 nlen 28 rlen 28, rlen-nlen 0, reclen 44 name <empty>
> [32778.081907]  offset 184 nlen 24 rlen 24, rlen-nlen 0, reclen 44 name <empty>
> [32778.089779]  offset 208 nlen 36 rlen 36, rlen-nlen 0, reclen 44 name <empty>
> [32778.097648]  offset 244 nlen 12 rlen 12, rlen-nlen 0, reclen 44 name REDACTED
> [32778.105227]  offset 256 nlen 24 rlen 24, rlen-nlen 0, reclen 44 name <empty>
> [32778.113099]  offset 280 nlen 24 rlen 24, rlen-nlen 0, reclen 44 name REDACTED
> [32778.122134]  offset 304 nlen 20 rlen 20, rlen-nlen 0, reclen 44 name REDACTED
> [32778.130780]  offset 324 nlen 16 rlen 16, rlen-nlen 0, reclen 44 name REDACTED
> [32778.138746]  offset 340 nlen 24 rlen 24, rlen-nlen 0, reclen 44 name <empty>
> [32778.146616]  offset 364 nlen 28 rlen 28, rlen-nlen 0, reclen 44 name <empty>
> [32778.154487]  offset 392 nlen 24 rlen 24, rlen-nlen 0, reclen 44 name <empty>
> [32778.162362]  offset 416 nlen 16 rlen 16, rlen-nlen 0, reclen 44 name <empty>
> ...
> 
> the file we were trying to insert needed a record length of 44, and none of the
> non-coalesced <empty> slots were big enough, so we failed and told do_split
> to get to work.
> 
> However, the sum of the non-empty entries didn't exceed half the block size, so
> the loop in do_split() iterated over all of the entries, ended at "count," and
> told us to split at (count - move) which is zero, and eventually:
> 
>        continued = hash2 == map[split - 1].hash;
> 
> exploded on the negative index.
> 
> It's an open question as to how this directory got into this format; I'm not
> sure if this should ever happen or not.  But at a minimum, I think we should
> be defensive here, hence [PATCH 1/1] will do that as an expedient fix and
> backportable patch for this situation.  There may be some other underlying
> probem which led to this directory structure if it's unexpected, and maybe that
> can come as another patch if anyone can investigate.

I thought this might be a bit of a conundrum.  There is *supposed* to be
merging of adjacent entries, but in some quick testing on RHEL7 (kernel
3.10.0-957.12.1.el7, same with Debian 4.14.79) shows this to be broken
if the files are deleted in dirent order (which would seem to be the most
common order):

# mkdir tmp; cd tmp
# touch file{1..100}
# rm file{33,36,37,39,41,42,43,46,47}
# debugfs -c -R "ls -ld tmp" /dev/sda1
   366  100644 (1)      0      0       0 18-Jun-2020 18:43 file30
<   369>      0 (1)      0      0   <reclen=  16> <deleted> file33
<   372>      0 (1)      0      0   <reclen=  16> <deleted> file36
<   373>      0 (1)      0      0   <reclen=  16> <deleted> file37
<   375>      0 (1)      0      0   <reclen=  16> <deleted> file39
<   377>      0 (1)      0      0   <reclen=  16> <deleted> file41
<   378>      0 (1)      0      0   <reclen=  16> <deleted> file42
<   379>      0 (1)      0      0   <reclen=  16> <deleted> file43
<   382>      0 (1)      0      0   <reclen=  16> <deleted> file46
<   383>      0 (1)      0      0   <reclen=  16> <deleted> file47
    386  100644 (1)      0      0       0 18-Jun-2020 18:43 file50

Above shows (with modified debugfs to show reclen for deleted files)
that the dirents are *not* combined.  If the dirent *before* the
other entries is deleted, then they are merged:

# rm file30
<   366>      0 (1)      0      0   <reclen= 160> <deleted> file30
<   369>      0 (1)      0      0   <reclen=  16> <deleted> file33
<   372>      0 (1)      0      0   <reclen=  16> <deleted> file36
<   373>      0 (1)      0      0   <reclen=  16> <deleted> file37
<   375>      0 (1)      0      0   <reclen=  16> <deleted> file39
<   377>      0 (1)      0      0   <reclen=  16> <deleted> file41
<   378>      0 (1)      0      0   <reclen=  16> <deleted> file42
<   379>      0 (1)      0      0   <reclen=  16> <deleted> file43
<   382>      0 (1)      0      0   <reclen=  16> <deleted> file46
<   383>      0 (1)      0      0   <reclen=  16> <deleted> file47
    386  100644 (1)      0      0       0 18-Jun-2020 18:43 file50

Cheers, Andreas






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