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

[Daniel Borkmann]

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.

>> causes a compiler error when building the perf tool in the linux-next tree.
>> I compile it using a FEDORA 28 installation, my gcc compiler version:
>> gcc (GCC) 8.0.1 20180324 (Red Hat 8.0.1-0.20)
>>
>> The file that causes the error is tools/lib/bpf/libbpf.c
>>
>> Here is the error message:
[...]
>> @@ -2334,7 +2331,7 @@ bpf_perf_event_read_simple(void *mem, unsigned long size,
>>  	__u64 data_tail = header->data_tail;
>>  	__u64 data_head = header->data_head;
>>  	void *base, *begin, *end;
>> -	int ret;
>> +	int ret = 0;
>>  
>>  	asm volatile("" ::: "memory"); /* in real code it should be smp_rmb() */
>>  	if (data_head == data_tail)
> 
> This looks like a separate issue.  The ret variable should really be
> enum bpf_perf_event_ret, so could you please initialize it to one of the
> values of this enum?
> 
> The uninitilized condition can only happen if (data_head != data_tail)
> but at the same time (data_head % size == data_tail % size) which
> should never really happen...  Perhaps initializing to
> LIBBPF_PERF_EVENT_ERROR would make sense?
> 
> Or better still adding:
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index f732237610e5..fa5a25945f19 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -2289,6 +2289,8 @@ bpf_perf_event_read_simple(void *mem, unsigned long size,
>  
>         begin = base + data_tail % size;
>         end = base + data_head % size;
> +       if (being == end)
> +               return LIBBPF_PERF_EVENT_ERROR;

Sounds good to me.

>         while (begin != end) {
>                 struct perf_event_header *ehdr;
>