[PATCH] libbpf: skip empty sections in bpf_object__init_global_data_maps

[PATCH] libbpf: skip empty sections in bpf_object__init_global_data_maps

[James Hilliard]

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.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Cc: Jose E. Marchesi <jose.marchesi@oracle.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

Re: [PATCH] libbpf: skip empty sections in bpf_object__init_global_data_maps

[Jiri Olsa]

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

jirka


[1] https://lore.kernel.org/bpf/YuKaFiZ+ksB5f0Ye@krava/

> 
> Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
> Cc: Jose E. Marchesi <jose.marchesi@oracle.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
> 

Re: [PATCH] libbpf: skip empty sections in bpf_object__init_global_data_maps

[David Faust]

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@gmail.com>
>> Cc: Jose E. Marchesi <jose.marchesi@oracle.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
>>

Re: [PATCH] libbpf: skip empty sections in bpf_object__init_global_data_maps

[Jiri Olsa]

On Mon, Aug 01, 2022 at 03:21:58PM -0700, David Faust wrote:
> 
> 
> 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

thanks, I'm acking this one then

Acked-by: Jiri Olsa <jolsa@kernel.org>

jirka

> 
> David
> 
> > 
> > jirka
> > 
> > 
> > [1] https://lore.kernel.org/bpf/YuKaFiZ+ksB5f0Ye@krava/
> > 
> >>
> >> Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
> >> Cc: Jose E. Marchesi <jose.marchesi@oracle.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
> >>

Re: [PATCH] libbpf: skip empty sections in bpf_object__init_global_data_maps

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <andrii@kernel.org>:

On Sun, 31 Jul 2022 17:26:49 -0600 you 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.
> 
> [...]

Here is the summary with links:
  - libbpf: skip empty sections in bpf_object__init_global_data_maps
    https://git.kernel.org/bpf/bpf-next/c/47ea7417b074

You are awesome, thank you!
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