[PATCH] keys: Fix proc_keys_next to increase position index

[Jarkko Sakkinen]

On Tue, Apr 14, 2020 at 09:33:16PM +0100, David Howells wrote:
> From: Vasily Averin <vvs at virtuozzo.com>
> 
> If seq_file .next function does not change position index,
> read after some lseek can generate unexpected output:
> 
> $ dd if=/proc/keys bs=1  # full usual output
> 0f6bfdf5 I--Q---     2 perm 3f010000  1000  1000 user      4af2f79ab8848d0a: 740
> 1fb91b32 I--Q---     3 perm 1f3f0000  1000 65534 keyring   _uid.1000: 2
> 27589480 I--Q---     1 perm 0b0b0000     0     0 user      invocation_id: 16
> 2f33ab67 I--Q---   152 perm 3f030000     0     0 keyring   _ses: 2
> 33f1d8fa I--Q---     4 perm 3f030000  1000  1000 keyring   _ses: 1
> 3d427fda I--Q---     2 perm 3f010000  1000  1000 user      69ec44aec7678e5a: 740
> 3ead4096 I--Q---     1 perm 1f3f0000  1000 65534 keyring   _uid_ses.1000: 1
> 521+0 records in
> 521+0 records out
> 521 bytes copied, 0,00123769 s, 421 kB/s
> 
> $ dd if=/proc/keys bs=500 skip=1  # read after lseek in middle of last line
> dd: /proc/keys: cannot skip to specified offset
> g   _uid_ses.1000: 1        <<<< end of last line
> 3ead4096 I--Q---     1 perm 1f3f0000  1000 65534 keyring   _uid_ses.1000: 1
>    <<<< and whole last line again
> 0+1 records in
> 0+1 records out
> 97 bytes copied, 0,000135035 s, 718 kB/s
> 
> $ dd if=/proc/keys bs=1000 skip=1   # read after lseek beyond end of file
> dd: /proc/keys: cannot skip to specified offset
> 3ead4096 I--Q---     1 perm 1f3f0000  1000 65534 keyring   _uid_ses.1000: 1
>    <<<< generates last line
> 0+1 records in
> 0+1 records out
> 76 bytes copied, 0,000119981 s, 633 kB/s
> 
> See https://bugzilla.kernel.org/show_bug.cgi?id=206283
> 
> Cc: stable at vger.kernel.org

# 4.19.x

> Fixes: 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code ...")
> Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
> Signed-off-by: David Howells <dhowells at redhat.com>

Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen at linux.intel.com>

/Jarkko