Re: [PATCH v2] perf build: Build error in libbpf with EXTRA_CFLAGS="-Wp,-D_FORTIFY_SOURCE=2 -O2"

[Daniel Borkmann]

On 07/27/2018 07:57 PM, Jakub Kicinski wrote:
> On Fri, 27 Jul 2018 09:22:03 +0200, Thomas-Mich Richter wrote:
>> On 07/27/2018 04:16 AM, Daniel Borkmann wrote:
>>> On 07/26/2018 03:48 AM, Jakub Kicinski wrote:  
>>>> On Wed, 25 Jul 2018 09:21:26 +0200, Thomas Richter wrote:  
>>>>> commit a5b8bd47dcc57 ("bpf tools: Collect eBPF programs from their own sections")  
>>>>
>>>> Hmm.. are you sure it's not 531b014e7a2f ("tools: bpf: make use of
>>>> reallocarray") that caused the issue?  That commit made us switch from
>>>> XSI-compliant to GNU-specific strerror_r() implementation..
>>>>
>>>> /me checks
>>>>
>>>> Yes it looks like 531b014e7a2f~ builds just fine.
>>>>
>>>> Daniel, did you try to apply v1 to the bpf tree?  Perhaps there is a
>>>> confusion about the trees here, if this is caused by my recent change
>>>> it's a bpf-next material.  strerror() works, but strerror_r() seems
>>>> nicer, so perhaps we could keep it if the patch worked in bpf-next?  
>>>
>>> Yeah indeed, the issue is only in bpf-next. When I compile libbpf from
>>> bpf tree with the below flags then it's all good> 
>>> Agree that we should rather use strerror_r() given this is a library.  
>>
>> Are you sure to stick with strerror_r? I ask because it is the only
>> occurence of strerror_r in this file. All other error messages use strerror
>> as in:
>> pr_warning("failed to create map (name: '%s'): %s\n",
>>                                    map->name,
>>                                    strerror(errno));
>>
>> $ fgrep strerror tools/lib/bpf/libbpf.c
>>                                         strerror(errno));
>>           issue I try to solve--->  strerror_r(-err, errmsg, sizeof(errmsg));
>>                                    map->name, strerror(errno), errno);
>>                                    strerror(errno));
>>         pr_warning("load bpf program failed: %s\n", strerror(errno));
>>                 pr_warning("failed to statfs %s: %s\n", dir, strerror(errno));
>>                 pr_warning("failed to pin program: %s\n", strerror(errno));
>>                 pr_warning("failed to mkdir %s: %s\n", path, strerror(-err));
>>                 pr_warning("failed to pin map: %s\n", strerror(errno));
>> $
>>
>> The next issue with strerror_r is to assign the return value to a variable.
>> Then we end up with variable set but not used:
>> libbpf.c: In function âbpf_object__elf_collectâ:
>> libbpf.c:809:35: error: variable âcpâ set but not used [-Werror=unused-but-set-variable]
>>      char errmsg[STRERR_BUFSIZE], *cp;
>>                                    ^
>> cc1: all warnings being treated as errors
> 
>        The GNU-specific strerror_r() returns a pointer to a string  containing
>        the  error  message.  This may be either a pointer to a string that the
>        function stores in buf, or a pointer to some (immutable) static  string
>        (in which case buf is unused).  If the function stores a string in buf,
>        then at most buflen bytes are stored (the string may  be  truncated  if
>        buflen is too small and errnum is unknown).  The string always includes
>        a terminating null byte ('\0').
> 
> IOW you gotta use the return value.
> 
>> Here is the source:
>>                        if (err) {
>>                                 char errmsg[STRERR_BUFSIZE], *cp;
>>
>>                                 cp = strerror_r(-err, errmsg, sizeof(errmsg));
>>                                 pr_warning("failed to alloc program %s (%s): %s",
>>                                            name, obj->path, errmsg);
>>                         }
>>
>> To fix this requires something like:
>>                                pr_warning("failed to alloc program %s (%s): %s",
>>                                            name, obj->path, cp);
> 
> This looks correct.
> 
>> And after pr_warning() is done, the local array errmsg is deleted.
>>
>> A lot of overkill in my opinion, unless I miss something.
> 
> IMO using potentially mutli-thread unsafe functions in a library is not
> optimal, we should strive to convert the other instances of strerror()
> rather than move the other way.

Yeah, fully agree. We should convert the other ones as well over to use
strerror_r().

Thanks,
Daniel