Skip to content
This repository has been archived by the owner on Dec 14, 2022. It is now read-only.

Commit

Permalink
tracing: Add barrier to trace_printk() buffer nesting modification
Browse files Browse the repository at this point in the history
commit 3d9622c upstream.

trace_printk() uses 4 buffers, one for each context (normal, softirq, irq
and NMI), such that it does not need to worry about one context preempting
the other. There's a nesting counter that gets incremented to figure out
which buffer to use. If the context gets preempted by another context which
calls trace_printk() it will increment the counter and use the next buffer,
and restore the counter when it is finished.

The problem is that gcc may optimize the modification of the buffer nesting
counter and it may not be incremented in memory before the buffer is used.
If this happens, and the context gets interrupted by another context, it
could pick the same buffer and corrupt the one that is being used.

Compiler barriers need to be added after the nesting variable is incremented
and before it is decremented to prevent usage of the context buffers by more
than one context at the same time.

Cc: Andy Lutomirski <[email protected]>
Fixes: e2ace00 ("tracing: Choose static tp_printk buffer by explicit nesting count")
Hat-tip-to: Peter Zijlstra <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
  • Loading branch information
rostedt authored and gregkh committed Sep 27, 2017
1 parent 100553e commit 96cf918
Showing 1 changed file with 7 additions and 1 deletion.
8 changes: 7 additions & 1 deletion kernel/trace/trace.c
Original file line number Diff line number Diff line change
Expand Up @@ -2369,11 +2369,17 @@ static char *get_trace_buf(void)
if (!buffer || buffer->nesting >= 4)
return NULL;

return &buffer->buffer[buffer->nesting++][0];
buffer->nesting++;

/* Interrupts must see nesting incremented before we use the buffer */
barrier();
return &buffer->buffer[buffer->nesting][0];
}

static void put_trace_buf(void)
{
/* Don't let the decrement of nesting leak before this */
barrier();
this_cpu_dec(trace_percpu_buffer->nesting);
}

Expand Down

0 comments on commit 96cf918

Please sign in to comment.