Re: [PATCH dwarves] btf_encoder: fix and complete filtering out zero-sized per-CPU variables

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Mon, May 24, 2021 at 04:42:22PM -0700, Andrii Nakryiko escreveu:
> btf_encoder is ignoring zero-sized per-CPU ELF symbols, but the same has to be
> done for DWARF variables when matching them with ELF symbols. This is due to
> zero-sized DWARF variables matching unrelated (non-zero-sized) variable that
> happens to be allocated at the exact same address, leading to a lot of
> confusion in BTF.

I've been following this, just didn't got to process it, will do it
soon.

- Arnaldo
 
> See [0] for when this causes big problems.
> 
>   [0] https://lore.kernel.org/bpf/CAEf4BzZ0-sihSL-UAm21JcaCCY92CqfNxycHRZYXcoj8OYb=wA@mail.gmail.com/
> 
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>  btf_encoder.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/btf_encoder.c b/btf_encoder.c
> index c711f124b31e..672b9943a4e2 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -538,6 +538,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
>  	cu__for_each_variable(cu, core_id, pos) {
>  		uint32_t size, type, linkage;
>  		const char *name, *dwarf_name;
> +		const struct tag *tag;
>  		uint64_t addr;
>  		int id;
>  
> @@ -550,6 +551,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
>  
>  		/* addr has to be recorded before we follow spec */
>  		addr = var->ip.addr;
> +		dwarf_name = variable__name(var, cu);
>  
>  		/* DWARF takes into account .data..percpu section offset
>  		 * within its segment, which for vmlinux is 0, but for kernel
> @@ -582,11 +584,9 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
>  		 *  modules per-CPU data section has non-zero offset so all
>  		 *  per-CPU symbols have non-zero values.
>  		 */
> -		if (var->ip.addr == 0) {
> -			dwarf_name = variable__name(var, cu);
> +		if (var->ip.addr == 0)
>  			if (!dwarf_name || strcmp(dwarf_name, name))
>  				continue;
> -		}
>  
>  		if (var->spec)
>  			var = var->spec;
> @@ -600,6 +600,13 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
>  			break;
>  		}
>  
> +		tag = cu__type(cu, var->ip.tag.type);
> +		if (tag__size(tag, cu) == 0) {
> +			if (btf_elf__verbose)
> +				fprintf(stderr, "Ignoring zero-sized per-CPU variable '%s'...\n", dwarf_name ?: "<missing name>");
> +			continue;
> +		}
> +
>  		type = var->ip.tag.type + type_id_off;
>  		linkage = var->external ? BTF_VAR_GLOBAL_ALLOCATED : BTF_VAR_STATIC;
>  
> -- 
> 2.30.2
> 

-- 

- Arnaldo