On Tue, Apr 14, 2020 at 09:33:16PM +0100, David Howells wrote: > From: Vasily Averin <vvs@xxxxxxxxxxxxx> > > 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@xxxxxxxxxxxxxxx # 4.19.x > Fixes: 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code ...") > Signed-off-by: Vasily Averin <vvs@xxxxxxxxxxxxx> > Signed-off-by: David Howells <dhowells@xxxxxxxxxx> Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx> /Jarkko
