linux-kernel - Re: [PATCH] libbpf: skip empty sections in bpf_object__init_global_data

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Date:   Mon, 1 Aug 2022 15:21:58 -0700
From:   David Faust <david.faust@...cle.com>
To:     Jiri Olsa <olsajiri@...il.com>,
        James Hilliard <james.hilliard1@...il.com>
Cc:     bpf@...r.kernel.org,
        "Jose E . Marchesi" <jose.marchesi@...cle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] libbpf: skip empty sections in
 bpf_object__init_global_data_maps



On 8/1/22 13:24, Jiri Olsa wrote:
> On Sun, Jul 31, 2022 at 05:26:49PM -0600, James Hilliard wrote:
>> The GNU assembler generates an empty .bss section. This is a well
>> established behavior in GAS that happens in all supported targets.
>>
>> The LLVM assembler doesn't generate an empty .bss section.
>>
>> bpftool chokes on the empty .bss section.
>>
>> Additionally in bpf_object__elf_collect the sec_desc->data is not
>> initialized when a section is not recognized. In this case, this
>> happens with .comment.
>>
>> So we must check that sec_desc->data is initialized before checking
>> if the size is 0.
> 
> oops David send same change but I asked him to move the check
> to bpf_object__elf_collect [1] .. but with your explanation this
> fix actualy looks fine to me

FWIW, I only just got back to actually making that change. This
patch has a much better explanation than the one I sent so +1 from
me also

David

> 
> jirka
> 
> 
> [1] https://lore.kernel.org/bpf/YuKaFiZ+ksB5f0Ye@krava/
> 
>>
>> Signed-off-by: James Hilliard <james.hilliard1@...il.com>
>> Cc: Jose E. Marchesi <jose.marchesi@...cle.com>
>> ---
>>  tools/lib/bpf/libbpf.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 50d41815f431..77e3797cf75a 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -1642,6 +1642,10 @@ static int bpf_object__init_global_data_maps(struct bpf_object *obj)
>>  	for (sec_idx = 1; sec_idx < obj->efile.sec_cnt; sec_idx++) {
>>  		sec_desc = &obj->efile.secs[sec_idx];
>>  
>> +		/* Skip recognized sections with size 0. */
>> +		if (sec_desc->data && sec_desc->data->d_size == 0)
>> +			continue;
>> +
>>  		switch (sec_desc->sec_type) {
>>  		case SEC_DATA:
>>  			sec_name = elf_sec_name(obj, elf_sec_by_idx(obj, sec_idx));
>> -- 
>> 2.34.1
>>