2022-03-11 21:05:43

by Jim Cromie

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

In https://lore.kernel.org/lkml/[email protected]/

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: [email protected]
Signed-off-by: Jim Cromie <[email protected]>

--
.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",
--
2.35.1

2022-03-11 21:14:10

by Jason Baron

Subject: 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/[email protected]/__;!!GjvTz_vk!HGKKoni4RVdEBgv_V0zPSNSX428bpf02zkCy2WbeQkBdVtp1QJqGX-lJYlRDGg$
>
> 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: [email protected]
> Signed-off-by: Jim Cromie <[email protected]>
>
> --
> .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 <[email protected]>

Thanks,

-Jason