Re: [PATCH 1/5] dyndbg: fix static_branch manipulation

On 3/10/22 23:47, Jim Cromie wrote:

In https://urldefense.com/v3/__https://lore.kernel.org/lkml/20211209150910.GA23...

Vincent's patch commented on, and worked around, a bug toggling static_branch's, when a 2nd PRINTK-ish flag was added. The bug results in a premature static_branch_disable when the 1st of 2 flags was disabled.

The cited commit computed newflags, but then in the JUMP_LABEL block, failed to use that result, instead using just one of the terms in it. Using newflags instead made the code work properly.

This is Vincents test-case, reduced. It needs the 2nd flag to work properly, but it's explanatory here.

pt_test() { echo 5 > /sys/module/dynamic_debug/verbose

site="module tcp" # just one callsite
echo " $site =_ " > /proc/dynamic_debug/control # clear it

# A B ~A ~B
for flg in +T +p "-T #broke here" -p; do

echo " $site $flg " > /proc/dynamic_debug/control done;

# A B ~B ~A
for flg in +T +p "-p #broke here" -T; do

echo " $site $flg " > /proc/dynamic_debug/control done } pt_test

Fixes: 84da83a6ffc0 dyndbg: combine flags & mask into a struct, simplify with it CC: vincent.whitchurch@axis.com Signed-off-by: Jim Cromie jim.cromie@gmail.com

-- .drop @stable, no exposed bug.

---

lib/dynamic_debug.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c index dd7f56af9aed..a56c1286ffa4 100644 --- a/lib/dynamic_debug.c +++ b/lib/dynamic_debug.c @@ -211,10 +211,11 @@ static int ddebug_change(const struct ddebug_query *query, continue; #ifdef CONFIG_JUMP_LABEL if (dp->flags & _DPRINTK_FLAGS_PRINT) {

• 		if (!(modifiers->flags & _DPRINTK_FLAGS_PRINT))
  

• 		if (!(newflags & _DPRINTK_FLAGS_PRINT))
  		static_branch_disable(&dp->key.dd_key_true);
  

• 	} else if (modifiers->flags & _DPRINTK_FLAGS_PRINT)
  

• 	} else if (newflags & _DPRINTK_FLAGS_PRINT) {
  	static_branch_enable(&dp->key.dd_key_true);
  

• 	}
  

#endif dp->flags = newflags; v4pr_info("changed %s:%d [%s]%s =%s\n",

Hi Jim,

If iiuc this is currently a bug but could be if we add a second 'print' bit such as for printing to the tracing logs. That said I agree that using 'newflags' here makes the code more straightforward/readable. So this one is fine with me.

Acked-by: Jason Baron jbaron@akamai.com

Thanks,

-Jason