Re: [PATCH dwarves] btf_encoder: verify 0 address DWARF variables are really in ELF section

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Dec 17, 2024 at 10:36:29AM +0000, Alan Maguire wrote:
> We use the DWARF location information to match a variable with its
> associated ELF section.  In the case of per-CPU variables their
> ELF section address range starts at 0, so any 0 address variables will
> appear to belong in that ELF section.  However, for "discard" sections
> DWARF encodes the associated variables with address location 0 so
> we need to double-check that address 0 variables really are in the
> associated section by checking the ELF symbol table.
> 
> This resolves an issue exposed by CONFIG_DEBUG_FORCE_WEAK_PER_CPU=y
> kernel builds where __pcpu_* dummary variables in a .discard section
> get misclassified as belonging in the per-CPU variable section since
> they specify location address 0.
> 
> Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>

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

thanks,
jirka


> ---
>  btf_encoder.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/btf_encoder.c b/btf_encoder.c
> index 3754884..04f547c 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -2189,6 +2189,26 @@ static bool filter_variable_name(const char *name)
>  	return false;
>  }
>  
> +bool variable_in_sec(struct btf_encoder *encoder, const char *name, size_t shndx)
> +{
> +	uint32_t sym_sec_idx;
> +	uint32_t core_id;
> +	GElf_Sym sym;
> +
> +	elf_symtab__for_each_symbol_index(encoder->symtab, core_id, sym, sym_sec_idx) {
> +		const char *sym_name;
> +
> +		if (sym_sec_idx != shndx || elf_sym__type(&sym) != STT_OBJECT)
> +			continue;
> +		sym_name = elf_sym__name(&sym, encoder->symtab);
> +		if (!sym_name)
> +			continue;
> +		if (strcmp(name, sym_name) == 0)
> +			return true;
> +	}
> +	return false;
> +}
> +
>  static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder)
>  {
>  	struct cu *cu = encoder->cu;
> @@ -2258,6 +2278,13 @@ static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder)
>  		if (filter_variable_name(name))
>  			continue;
>  
> +		/* A 0 address may be in a "discard" section; DWARF provides
> +		 * location information with address 0 for such variables.
> +		 * Ensure the variable really is in this section by checking
> +		 * the ELF symtab.
> +		 */
> +		if (addr == 0 && !variable_in_sec(encoder, name, shndx))
> +			continue;
>  		/* Check for invalid BTF names */
>  		if (!btf_name_valid(name)) {
>  			dump_invalid_symbol("Found invalid variable name when encoding btf",
> -- 
> 2.31.1
>