Re: [PATCH] printk: Fix discarding of records

[Linus Torvalds]

Adding Kay and Greg, since the original code is from commit
7ff9554bb578 ("printk: convert byte-buffer to variable-length record
buffer") and all the "prev" flag tweaks end up building on top of
that.

The whole "prev flags" is messed up, and LOG_CONT is done very confusingly.

Why are *those* particular two "prev = msg->flags" incorrect, when
every other case where we walk the messages they are required?

The code/logic makes no sense. You remove the "prev = msg->flags" at
line 1070, when the *identical* loop just above it has it. So now the
two loops count the number of characters differently. That makes no
sense.

So I don't think this fixes the fundamental problem. I'm more inclined
to believe that LOG_CONT is wrongly set somewhere, for example because
a continuation wasn't actually originally printed due to coming from
different users or something like that.

Or at the very least I want a coherent explanation why one loop would
do this and the other would not, and why counting up *different*
numbers could possibly make sense.

Because as it is, there clearly is some problem, but the patch does
not look sensible to me.

                 Linus

On Thu, Feb 13, 2014 at 8:42 PM, Debabrata Banerjee <dbanerje@xxxxxxxxxx> wrote:
> Found another buffer overflow in this code that was introduced by
> e3756477aec028427fec767957c0d4b6cfb87208 trying to solve a related overflow.
>
> strace still shows a problem:
>
>         syslog(0x3, 0x7fffd65375d0, 0x1000)     = 4107
>
> The first record output was in the middle of a LOG_CONT line:
>
>         <4>[    2.974999] 0x0000000000000500-0x000000000000052f SystemIO conflicts with Region \GPIO 1 (20130328/utaddress-251)
>
> This happens because when discarding records to be less than len, every line it
> subtracts should be the first line which means prev should be 0. Otherwise for
> example it won't include the prefix len if the last line was a LOG_CONT
> record, which however will be printed because there won't be a previous line.
> This patch makes sure enough records are discarded to be under len.
>
> CC: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Debabrata Banerjee <dbanerje@xxxxxxxxxx>
> ---
>  kernel/printk/printk.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index b1d255f..c1722d5 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1067,7 +1067,6 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
>                         struct printk_log *msg = log_from_idx(idx);
>
>                         len -= msg_print_text(msg, prev, true, NULL, 0);
> -                       prev = msg->flags;
>                         idx = log_next(idx);
>                         seq++;
>                 }
> @@ -2780,7 +2779,6 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
>                 l -= msg_print_text(msg, prev, true, NULL, 0);
>                 idx = log_next(idx);
>                 seq++;
> -               prev = msg->flags;
>         }
>
>         /* last message in next interation */
> --
> 1.8.3.4
>
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