Re: [PATCH v4 05/13] module: Add printk formats to add module build ID to stacktraces

[Andy Shevchenko]

On Mon, Apr 12, 2021 at 12:29:05PM -0700, Stephen Boyd wrote:
> Quoting Andy Shevchenko (2021-04-12 04:58:02)
> > On Fri, Apr 09, 2021 at 06:52:52PM -0700, Stephen Boyd wrote:
> > > Let's make kernel stacktraces easier to identify by including the build
> > > ID[1] of a module if the stacktrace is printing a symbol from a module.
> > > This makes it simpler for developers to locate a kernel module's full
> > > debuginfo for a particular stacktrace. Combined with
> > > scripts/decode_stracktrace.sh, a developer can download the matching
> > > debuginfo from a debuginfod[2] server and find the exact file and line
> > > number for the functions plus offsets in a stacktrace that match the
> > > module. This is especially useful for pstore crash debugging where the
> > > kernel crashes are recorded in something like console-ramoops and the
> > > recovery kernel/modules are different or the debuginfo doesn't exist on
> > > the device due to space concerns (the debuginfo can be too large for
> > > space limited devices).
> > > 
> > > Originally, I put this on the %pS format, but that was quickly rejected
> > > given that %pS is used in other places such as ftrace where build IDs
> > > aren't meaningful. There was some discussions on the list to put every
> > > module build ID into the "Modules linked in:" section of the stacktrace
> > > message but that quickly becomes very hard to read once you have more
> > > than three or four modules linked in. It also provides too much
> > > information when we don't expect each module to be traversed in a
> > > stacktrace. Having the build ID for modules that aren't important just
> > > makes things messy. Splitting it to multiple lines for each module
> > > quickly explodes the number of lines printed in an oops too, possibly
> > > wrapping the warning off the console. And finally, trying to stash away
> > > each module used in a callstack to provide the ID of each symbol printed
> > > is cumbersome and would require changes to each architecture to stash
> > > away modules and return their build IDs once unwinding has completed.
> > > 
> > > Instead, we opt for the simpler approach of introducing new printk
> > > formats '%pS[R]b' for "pointer symbolic backtrace with module build ID"
> > > and '%pBb' for "pointer backtrace with module build ID" and then
> > > updating the few places in the architecture layer where the stacktrace
> > > is printed to use this new format.
> > > 
> > > Example:
> > 
> > Can you trim a bit the example, so we will see only important lines.
> > In such case you may provide "before" and "after" variants.
> > 
> > ...
> > 
> > > -     if (modname)
> > > -             len += sprintf(buffer + len, " [%s]", modname);
> > > +     if (modname) {
> > > +             len += sprintf(buffer + len, " [%s", modname);
> > 
> > > +             /* build ID should match length of sprintf below */
> > > +             BUILD_BUG_ON(BUILD_ID_SIZE_MAX != 20);
> > 
> > First of all, why not static_assert() defined near to the actual macro?
> 
> Which macro? BUILD_ID_SIZE_MAX?

Yes.

> I tried static_assert() and it didn't
> work for me but maybe I missed something.

Sounds weird. static_assert() is a good one. Check, for example, lib/vsprintf.c
on how to use it.

> Why is static_assert()
> preferred?

Because it's cleaner way to achieve it and as a bonus it can be put outside of
the functions (be in the header or so).

> > > +             if (IS_ENABLED(CONFIG_STACKTRACE_BUILD_ID) && add_buildid && buildid)
> > > +                     len += sprintf(buffer + len, " %20phN", buildid);
> > 
> >                         len += sprintf(buffer + len, " %*phN", BUILD_ID_SIZE_MAX, buildid);
> > 
> 
> Are you suggesting to use sprintf format here so that the size is part
> of the printf? Sounds good to me. Thanks.

I prefer %20phN when the size is carved in stone (for example by
specification), but if you are really expecting that it may be
changed in the future, use variadic approach as I showed above.

-- 
With Best Regards,
Andy Shevchenko