From: Vasily Averin vvs@virtuozzo.com
[ Upstream commit 10c8d69f314d557d94d74ec492575ae6a4f1eb1c ]
If seq_file .next fuction does not change position index, read after some lseek can generate unexpected output.
In Aug 2018 NeilBrown noticed commit 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface") "Some ->next functions do not increment *pos when they return NULL... Note that such ->next functions are buggy and should be fixed. A simple demonstration is
dd if=/proc/swaps bs=1000 skip=1
Choose any block size larger than the size of /proc/swaps. This will always show the whole last line of /proc/swaps"
Described problem is still actual. If you make lseek into middle of last output line following read will output end of last line and whole last line once again.
$ dd if=/proc/swaps bs=1 # usual output Filename Type Size Used Priority /dev/dm-0 partition 4194812 97536 -2 104+0 records in 104+0 records out 104 bytes copied
$ dd if=/proc/swaps bs=40 skip=1 # last line was generated twice dd: /proc/swaps: cannot skip to specified offset v/dm-0 partition 4194812 97536 -2 /dev/dm-0 partition 4194812 97536 -2 3+1 records in 3+1 records out 131 bytes copied
https://bugzilla.kernel.org/show_bug.cgi?id=206283
Link: http://lkml.kernel.org/r/bd8cfd7b-ac95-9b91-f9e7-e8438bd5047d@virtuozzo.com Signed-off-by: Vasily Averin vvs@virtuozzo.com Reviewed-by: Andrew Morton akpm@linux-foundation.org Cc: Jann Horn jannh@google.com Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: Kees Cook keescook@chromium.org Cc: Hugh Dickins hughd@google.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- mm/swapfile.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/swapfile.c b/mm/swapfile.c index 891a3ef486511..646fd0a8e3202 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -2737,10 +2737,10 @@ static void *swap_next(struct seq_file *swap, void *v, loff_t *pos) else type = si->type + 1;
+ ++(*pos); for (; (si = swap_type_to_swap_info(type)); type++) { if (!(si->flags & SWP_USED) || !si->swap_map) continue; - ++*pos; return si; }