On Tue, 26 Aug 2014 11:19:41 +0200
Petr Mládek <pmla...@suse.cz> wrote:
> 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()).
>
> Great catch! And I agree with Jack about that strlen.
Although I will say that ARRAY_SIZE() is used elsewhere in the kernel
like this.
But as this isn't a critical section, either is fine.
Acked-by: Steven Rostedt <rost...@goodmis.org>
-- Steve
>
> > 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);
> > }
> >
> > /*
> > --
> > 2.1.0
> >
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