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.
 
> See [0] for when this causes big problems.
 
>   [0] https://lore.kernel.org/bpf/CAEf4BzZ0-sihSL-UAm21JcaCCY92CqfNxycHRZYXcoj8OYb=wA@mail.gmail.com/

> +++ b/btf_encoder.c
> @@ -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,

I just changed the above hunk to be:

@@ -583,7 +585,6 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
                 *  per-CPU symbols have non-zero values.
                 */
                if (var->ip.addr == 0) {
-                       dwarf_name = variable__name(var, cu);
                        if (!dwarf_name || strcmp(dwarf_name, name))
                                continue;
                }


Which is shorter and keeps the {} around a multi line if block, ok?

Thanks, applied!

- Arnaldo