[PATCH] [libbacktrace] Initialize st in elf_is_symlink

[Jeff Law]

On 3/18/19 6:35 PM, James Hilliard wrote:
> On Mon, Mar 18, 2019 at 5:46 PM Jeff Law <law@redhat.com> wrote:
>>
>> On 3/18/19 5:07 PM, James Hilliard wrote:
>>> On Mon, Mar 18, 2019 at 4:51 PM Jakub Jelinek <jakub@redhat.com> wrote:
>>>>
>>>> On Mon, Mar 18, 2019 at 04:41:05PM -0600, James Hilliard wrote:
>>>>>> Thanks, but I'm saying that if you look at the code you can see that
>>>>>> st is clearly initialized, by the call to lstat.  I would like to see
>>>>>> an explanation for why you are seeing that warning before changing the
>>>>>> code to disable it.  Initializing st should not be necessary here.
>>>>>> For example, perhaps lstat is a macro when compiling libsanitizer; if
>>>>>> that is the underlying problem, then we should fix the macro, not this
>>>>>> code.
>>>>> Yeah, I'm not sure why the compiler thinks lstat isn't initializing st.
>>>>> What should I do to debug this further?
>>>>
>>>> Guess you should start by telling us which OS it is on (I can't reproduce
>>>> this warning on x86_64-linux nor i686-linux with glibc 2.28), looking at
>>>> preprocessed source to see what exactly lstat does (e.g. if it is some macro
>>>> or inline function and what exactly it is doing).
>>> I am cross compiling with buildroot master branch using ubuntu 18.10.
>>> I am building gcc 8.3.0 and glibc 2.29 for the cross toolchain.
>>> The build and target systems are both x86_64.
>> Add "-save-temps" to the command line.  That will create a .i file, send
>> the .i file along with the command line.
> I added --save-temps to the cflags for the gcc package build.
> Here's the command line log:
> https://gist.githubusercontent.com/jameshilliard/015d972c26d1ec7bbbd1ad2d57d5dd3b/raw/7c848cdfba0967a681401de24ed6f4c86315d2d9/cli.log
> Here's the elf.i fille:
> https://gist.githubusercontent.com/jameshilliard/2a391d7292f3ba7412fe80e166c0f0e3/raw/63eb16ba9eff0b3ee668b16257b0948b2ca894a0/elf.i
Jakub -- I haven't looked at it in any detail, but the first thing that
jumps out at me is the -Og and the message:

> ../../../../libsanitizer/libbacktrace/../../libbacktrace/elf.c: In function ‘elf_is_symlink’:
> ../../../../libsanitizer/libbacktrace/../../libbacktrace/elf.c:772:21: error: ‘st.st_mode’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
>    return S_ISLNK (st.st_mode);

It could be the issues we've see with SRA and aggregates that have led
to the discussion around breaking those into a distinct class of uninit
warnings because we don't handle them well.  I wouldn't be at all
surprised if it goes away at -O1 or -O2.

jeff