On Sat, Nov 26, 2022 at 12:51:53PM +0900, Takashi Sakamoto wrote: > Hi, > > On Fri, Nov 25, 2022 at 04:23:26PM +0000, John Keeping wrote: > > Strings need to be specially marked in trace events to ensure the > > content is captured, othewise the trace just shows the value of the > > pointer. > > > > Signed-off-by: John Keeping <john@xxxxxxxxxxxx> > > --- > > sound/core/pcm_trace.h | 6 +++--- > > 1 file changed, 3 insertions(+), 3 deletions(-) > > Thanks for the patch, while I have a slight concern about the intension > of change. > > Let's see message in below commit to add 'trace_safe_str()' in > 'kernel/trace/trace.c': > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9a6944fee68 > > "The proper way to handle displaying any string that is not guaranteed to be > in the kernel core rodata section, is to copy it into the ring buffer via > the __string(), __assign_str() and __get_str() helper macros." > > Additionally, the macros looks to be alias to __dynamic_array() or so. > > In our case, the pointer of 'why' argument should points to two > hardcoded strings; "hw_ptr skipping" and "Unexpected hw_ptr". As long as > I know, they are put into any of .rodata section. > > ``` > $ objdump -s sound/core/snd-pcm.ko -j .rodata.str1.1 > ``` > 0830 74757265 20436861 6e6e656c 204d6170 ture Channel Map > 0840 00585255 4e3a2025 730a0055 6e657870 .XRUN: %s..Unexp > 0850 65637465 64206877 5f707472 0068775f ected hw_ptr.hw_ > 0860 70747220 736b6970 70696e67 004c6f73 ptr skipping.Los > 0870 7420696e 74657272 75707473 3f00756e t interrupts?.un > ``` > > I think current implementation is enough safe. Thanks for this analysis - I hadn't looked into the history of this. It seems that trace-cmd's record/report functionality does not support rodata strings in this way though. Without this patch, the trace log is not very informative: $ trace-cmd record -e snd_pcm:hw_ptr_error ^C $ trace-cmd report irq/49-ehci_hcd-111 [002] 65.785147: hw_ptr_error: pcmC1D0p/sub0: ERROR: c0b1b3c7 With this patch applied this becomes: irq/49-ehci_hcd-111 [002] 435.758754: hw_ptr_error: pcmC2D0p/sub0: ERROR: Lost interrupts? > Nevertheless, explicit usage of the macros are developer friendly in my > opinion since string buffer in C language tends to bring problems and > the usage of macro would reduce careless future mistakes. In the case, > when probing the event, the string is copied to memory for event > structure, thus it adds slight overhead than current implementation. I > leave the decision to maintainer. > > To maintainer, if you apply the patch to your tree, feel free to add my > review tag. > > Reviewed-by: Takashi Sakamoto <o-takashi@xxxxxxxxxxxxx> > > > diff --git a/sound/core/pcm_trace.h b/sound/core/pcm_trace.h > > index f18da2050772..350b40b906ca 100644 > > --- a/sound/core/pcm_trace.h > > +++ b/sound/core/pcm_trace.h > > @@ -88,19 +88,19 @@ TRACE_EVENT(hw_ptr_error, > > __field( unsigned int, device ) > > __field( unsigned int, number ) > > __field( unsigned int, stream ) > > - __field( const char *, reason ) > > + __string( reason, why ) > > ), > > TP_fast_assign( > > __entry->card = (substream)->pcm->card->number; > > __entry->device = (substream)->pcm->device; > > __entry->number = (substream)->number; > > __entry->stream = (substream)->stream; > > - __entry->reason = (why); > > + __assign_str(reason, why); > > ), > > TP_printk("pcmC%dD%d%s/sub%d: ERROR: %s", > > __entry->card, __entry->device, > > __entry->stream == SNDRV_PCM_STREAM_PLAYBACK ? "p" : "c", > > - __entry->number, __entry->reason) > > + __entry->number, __get_str(reason)) > > ); > > > > TRACE_EVENT(applptr, > > -- > > 2.38.1
