Re: cannot skip augment string handling

[Mark Wielaard <mark@klomp.org>]

Hi Ulrich,

On Tue, Aug 09, 2022 at 08:01:43PM +0200, Ulrich Drepper via Elfutils-devel wrote:
> He dwarf_next_cfi function has some clever code which skips over the
> processing of the augmentation string content if the first character is 'z'
> (for sized augmentation).  This would be OK if it wouldn't be for the fact
> that the augment processing loop produces additional information, namely,
> it fills in the fde_augmentation_data_size fields.  That information isn't
> available elsewhere.
> 
> In addition, the loop over the augment string is incorrect because the
> interpretation of the P, L, and R entries depends on 'z' being present.  in
> the absence of 'z', when the loop would be executed in the current version,
> the interpretation of those entries is not the same.
> 
> In the patch below I've removed the shortcut and fixed the handling of the
> P, L, and R entries.  I've also added an additional test checking that the
> entries of the augmentation string don't guide the code to consume more
> data then is indicated in the 'z' data.

Looks good. Thanks for catching this. Please do add a Signed-off-by
line next time. See the CONTRIBUTING file.

I was wondering why this hasn't caused an issue before. But it looks
like internally when we use the result of dwarf_next_cfi in cie.c and
fde.c we always call __libdw_intern_cie or intern_new_cie which
recalculates the fde_augmentation_data_size by reading the
augmentation string and data again.

Thanks,

Mark