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

[Jiri Olsa <olsajiri@gmail.com>]

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>

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
>