On Sun 24-08-14 12:01:36, Patrick Palka wrote:
> We shouldn't set text_len in the code path that detects printk recursion
> because text_len corresponds to the length of the string inside textbuf.
> A few lines down from the line
>
> text_len = strlen(recursion_msg);
>
> is the line
>
> text_len += vscnprintf(text + text_len, ...);
>
> So if printk detects recursion, it sets text_len to 29 and logs an
> error. Then the message supplied by the caller of printk is stored
> inside textbuf but offset by 29 bytes. This means that the output of
> the recursive call to printk will contain 29 bytes of garbage in front
> of it.
>
> This defect is caused by commit 458df9fd ("printk: remove separate
> printk_sched buffers and use printk buf instead") which turned
>
> text_len = vscnprintf(text + text_len, ...);
>
> into
>
> text_len += vscnprintf(text + text_len, ...);
>
> To fix this, this patch avoids setting text_len when logging the printk
> recursion error. This patch also performs a couple of local
> micro-optimizations (use unlikely() and ARRAY_SIZE()).
Thanks for spotting this! Just one nit below:
> Signed-off-by: Patrick Palka <patr...@parcs.ath.cx>
> ---
> kernel/printk/printk.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index e04c455..c101ec2 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1665,15 +1665,15 @@ asmlinkage int vprintk_emit(int facility, int level,
> raw_spin_lock(&logbuf_lock);
> logbuf_cpu = this_cpu;
>
> - if (recursion_bug) {
> + if (unlikely(recursion_bug)) {
> static const char recursion_msg[] =
> "BUG: recent printk recursion!";
>
> recursion_bug = 0;
> - text_len = strlen(recursion_msg);
> /* emit KERN_CRIT message */
> printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0,
> - NULL, 0, recursion_msg, text_len);
> + NULL, 0, recursion_msg,
> + ARRAY_SIZE(recursion_msg) - 1);
I'd prefer to keep using strlen(recursion_msg). It is more obvious what
it does and is more futureproof.
Honza
--
Jan Kara <j...@suse.cz>
SUSE Labs, CR
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