Re: [PATCH v2 dwarves 0/2] Check DW_OP_[GNU_]entry_value for possible parameter matching

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Nov 15, 2024 at 11:52:38AM -0300, Arnaldo Carvalho de Melo wrote:
> On Fri, Nov 15, 2024 at 10:34:35AM +0100, Jiri Olsa wrote:
> > On Thu, Nov 14, 2024 at 03:58:20PM +0000, Alan Maguire wrote:
> > > Currently, pahole relies on DWARF to find whether a particular func
> > > has its parameter mismatched with standard or optimized away.
> > > In both these cases, the func will not be put in BTF and this
> > > will prevent fentry/fexit tracing for these functions.
> > > 
> > > The current parameter checking focuses on the first location/expression
> > > to match intended parameter register. But in some cases, the first
> > > location/expression does not have expected matching information,
> > > but further location like DW_OP_[GNU_]entry_value can provide
> > > information which matches the expected parameter register.
> > > 
> > > Patch 1 supports this; patch 2 adds locking around dwarf_getlocation*
> > > as it is unsafe in a multithreaded environment.
> > > 
> > > Run ~4000 times without observing a segmentation fault (as compared
> > > to without patch 2, where a segmentation fault is observed approximately
> > > every 200 invokations).
> > > 
> > > Changes since v1:
> > > 
> > > - used Eduard's approach of using a __dwarf_getlocations()
> > >   internal wrapper (Eduard, patch 1).
> > > - renamed function to parameter__reg(); did not rename
> > >   __dwarf_getlocations() since its functionality is based around
> > >   retrieving DWARF location info rather than parameter register
> > >   indices (Yonghong, patch 2)
> > > - added locking around dwarf_getlocation*() usage in dwarf_loader
> > >   to avoid segmentation faults reported by Eduard (Jiri, Arnaldo,
> > >   patch 2)
> > 
> > looks good, I got 95 more functions in clang build including perf_event_read
> > and there's no change in generated functions with gcc build
> > 
> > Acked-by: Jiri Olsa <jolsa@kernel.org>
> 
> I think since you tested it we can stick this, right?
> 
> Tested-by: Jiri Olsa <jolsa@kernel.org>

yep, sure

jirka

>  
> > thanks,
> > jirka
> > 
> > > 
> > > Alan Maguire (1):
> > >   dwarf_loader: use libdw__lock for dwarf_getlocation(s)
> > > 
> > > Eduard Zingerman (1):
> > >   dwarf_loader: Check DW_OP_[GNU_]entry_value for possible parameter
> > >     matching
> > > 
> > >  dwarf_loader.c | 121 +++++++++++++++++++++++++++++++++++++++----------
> > >  1 file changed, 96 insertions(+), 25 deletions(-)
> > > 
> > > -- 
> > > 2.31.1
> > >