2020-11-26 08:21:27

by Minchan Kim

Subject: [PATCH] tracing: Fix align of static buffer

With 5.9 kernel on ARM64, I found ftrace_dump output was broken but
it had no problem with normal output "cat /sys/kernel/debug/tracing/trace".

With investigation, it seems coping the data into temporal buffer seems to
break the align binary printf expects if the static buffer is not aligned
with 4-byte. IIUC, get_arg in bstr_printf expects that args has already
right align to be decoded and seq_buf_bprintf says ``the arguments are saved
in a 32bit word array that is defined by the format string constraints``.
So if we don't keep the align under copy to temporal buffer, the output
will be broken by shifting some bytes.

This patch fixes it.

Cc: <[email protected]>
Fixes: 8e99cf91b99bb ("tracing: Do not allocate buffer in trace_find_next_entry() in atomic")
Signed-off-by: Namhyung Kim <[email protected]>
Signed-off-by: Minchan Kim <[email protected]>
---
kernel/trace/trace.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 6a282bbc7e7f..01bfcc345d55 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3534,7 +3534,7 @@ __find_next_entry(struct trace_iterator *iter, int *ent_cpu,
}

#define STATIC_TEMP_BUF_SIZE 128
-static char static_temp_buf[STATIC_TEMP_BUF_SIZE];
+static char static_temp_buf[STATIC_TEMP_BUF_SIZE] __aligned(4);

/* Find the next real entry, without updating the iterator itself */
struct trace_entry *trace_find_next_entry(struct trace_iterator *iter,
--
2.29.2.454.gaff20da3a2-goog

2020-11-26 09:45:55

by Greg Kroah-Hartman

Subject: Re: [PATCH] tracing: Fix align of static buffer

On Wed, Nov 25, 2020 at 02:56:54PM -0800, Minchan Kim wrote:
> With 5.9 kernel on ARM64, I found ftrace_dump output was broken but
> it had no problem with normal output "cat /sys/kernel/debug/tracing/trace".
>
> With investigation, it seems coping the data into temporal buffer seems to
> break the align binary printf expects if the static buffer is not aligned
> with 4-byte. IIUC, get_arg in bstr_printf expects that args has already
> right align to be decoded and seq_buf_bprintf says ``the arguments are saved
> in a 32bit word array that is defined by the format string constraints``.
> So if we don't keep the align under copy to temporal buffer, the output
> will be broken by shifting some bytes.
>
> This patch fixes it.

Does this resolve the issue reported at:
https://lore.kernel.org/r/[email protected]
?

thanks,

greg k-h

2020-11-26 12:44:32

by Youngmin Nam

Subject: Re: [PATCH] tracing: Fix align of static buffer

Hi Minchan,

Feel free to add my:

Tested-by: Youngmin Nam <[email protected]>

2020-11-26 18:09:07

by Minchan Kim

Subject: Re: [PATCH] tracing: Fix align of static buffer

On Thu, Nov 26, 2020 at 10:04:28PM +0900, Youngmin Nam wrote:
> Hi Minchan,
>
> Feel free to add my:
>
> Tested-by: Youngmin Nam <[email protected]>

Thanks for the testing, Youngmin!

2020-11-27 05:52:37

by Namhyung Kim

Subject: Re: [PATCH] tracing: Fix align of static buffer

Hi Greg,

On Thu, Nov 26, 2020 at 2:52 PM Greg KH <[email protected]> wrote:
>
> On Wed, Nov 25, 2020 at 02:56:54PM -0800, Minchan Kim wrote:
> > With 5.9 kernel on ARM64, I found ftrace_dump output was broken but
> > it had no problem with normal output "cat /sys/kernel/debug/tracing/trace".
> >
> > With investigation, it seems coping the data into temporal buffer seems to
> > break the align binary printf expects if the static buffer is not aligned
> > with 4-byte. IIUC, get_arg in bstr_printf expects that args has already
> > right align to be decoded and seq_buf_bprintf says ``the arguments are saved
> > in a 32bit word array that is defined by the format string constraints``.
> > So if we don't keep the align under copy to temporal buffer, the output
> > will be broken by shifting some bytes.
> >
> > This patch fixes it.
>
> Does this resolve the issue reported at:
> https://lore.kernel.org/r/[email protected]
> ?

No, it's a different issue.

Thanks,
Namhyung