Re: [PATCH bpf-next v1 1/3] tools/resolve_btfids: Simplify handling cross-endian compilation

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 9/20/24 09:49, Tony Ambardar wrote:
> Initially, the .BTF_ids section was created zero-filled and then patched
> with BTF IDs by resolve_btfids on the build host. Patching was done in
> native endianness and thus failed to work for cross-endian compile targets.
> This was fixed in [1] by using libelf-based translation to output patched
> data in target byte order.
> 
> The addition of 8-byte BTF sets in [2] lead to .BTF_ids creation with both
> target-endian values and zero-filled data to be later patched. This again
> broke cross-endian compilation as the already-correct target-endian values
> were translated on output by libelf [1]. The problem was worked around [3]
> by manually converting BTF SET8 values to native endianness, so that final
> libelf output translation yields data in target byte order.
> 
> Simplify and make the code more robust against future changes like [2] by
> employing libelf-based endian translation on both input and output, which
> is typical of libelf usage.
> 
> [1]: 61e8aeda9398 ("bpf: Fix libelf endian handling in resolv_btfids")
> [2]: ef2c6f370a63 ("tools/resolve_btfids: Add support for 8-byte BTF sets")
> [3]: 903fad439466 ("tools/resolve_btfids: Fix cross-compilation to non-host endianness")
> 
> CC: Viktor Malik <vmalik@xxxxxxxxxx>
> Signed-off-by: Tony Ambardar <tony.ambardar@xxxxxxxxx>
> ---

Acked-by: Viktor Malik <vmalik@xxxxxxxxxx>

>  tools/bpf/resolve_btfids/main.c | 60 ++++++++++++---------------------
>  1 file changed, 22 insertions(+), 38 deletions(-)
> 
> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
> index d54aaa0619df..9f1ab23ed014 100644
> --- a/tools/bpf/resolve_btfids/main.c
> +++ b/tools/bpf/resolve_btfids/main.c
> @@ -90,14 +90,6 @@
>  
>  #define ADDR_CNT	100
>  
> -#if __BYTE_ORDER == __LITTLE_ENDIAN
> -# define ELFDATANATIVE	ELFDATA2LSB
> -#elif __BYTE_ORDER == __BIG_ENDIAN
> -# define ELFDATANATIVE	ELFDATA2MSB
> -#else
> -# error "Unknown machine endianness!"
> -#endif
> -
>  struct btf_id {
>  	struct rb_node	 rb_node;
>  	char		*name;
> @@ -125,7 +117,6 @@ struct object {
>  		int		 idlist_shndx;
>  		size_t		 strtabidx;
>  		unsigned long	 idlist_addr;
> -		int		 encoding;
>  	} efile;
>  
>  	struct rb_root	sets;
> @@ -325,11 +316,30 @@ static int compressed_section_fix(Elf *elf, Elf_Scn *scn, GElf_Shdr *sh)
>  	return 0;
>  }
>  
> +static int btfids_endian_fix(struct object *obj)
> +{
> +	Elf_Data *btfids = obj->efile.idlist;
> +	Elf *elf = obj->efile.elf;
> +	int file_byteorder;
> +
> +	/* This should always succeed due to prior ELF checks */
> +	file_byteorder = elf_getident(elf, NULL)[EI_DATA];
> +
> +	/* Set type to ensure endian translation occurs, and manually invoke
> +	 * translation on input since .BTF_ids section as created disables it.
> +	 */
> +	btfids->d_type = ELF_T_WORD;
> +	if (gelf_xlatetom(elf, btfids, btfids, file_byteorder) == NULL) {
> +		pr_err("FAILED xlatetom .BTF_ids data: %s\n", elf_errmsg(-1));
> +		return -1;
> +	}
> +	return 0;
> +}
> +
>  static int elf_collect(struct object *obj)
>  {
>  	Elf_Scn *scn = NULL;
>  	size_t shdrstrndx;
> -	GElf_Ehdr ehdr;
>  	int idx = 0;
>  	Elf *elf;
>  	int fd;
> @@ -361,13 +371,6 @@ static int elf_collect(struct object *obj)
>  		return -1;
>  	}
>  
> -	if (gelf_getehdr(obj->efile.elf, &ehdr) == NULL) {
> -		pr_err("FAILED cannot get ELF header: %s\n",
> -			elf_errmsg(-1));
> -		return -1;
> -	}
> -	obj->efile.encoding = ehdr.e_ident[EI_DATA];
> -
>  	/*
>  	 * Scan all the elf sections and look for save data
>  	 * from .BTF_ids section and symbols.
> @@ -409,6 +412,8 @@ static int elf_collect(struct object *obj)
>  			obj->efile.idlist       = data;
>  			obj->efile.idlist_shndx = idx;
>  			obj->efile.idlist_addr  = sh.sh_addr;
> +			if (btfids_endian_fix(obj))
> +				return -1;
>  		} else if (!strcmp(name, BTF_BASE_ELF_SEC)) {
>  			/* If a .BTF.base section is found, do not resolve
>  			 * BTF ids relative to vmlinux; resolve relative
> @@ -706,24 +711,6 @@ static int sets_patch(struct object *obj)
>  			 */
>  			BUILD_BUG_ON((u32 *)set8->pairs != &set8->pairs[0].id);
>  			qsort(set8->pairs, set8->cnt, sizeof(set8->pairs[0]), cmp_id);
> -
> -			/*
> -			 * When ELF endianness does not match endianness of the
> -			 * host, libelf will do the translation when updating
> -			 * the ELF. This, however, corrupts SET8 flags which are
> -			 * already in the target endianness. So, let's bswap
> -			 * them to the host endianness and libelf will then
> -			 * correctly translate everything.
> -			 */
> -			if (obj->efile.encoding != ELFDATANATIVE) {
> -				int i;
> -
> -				set8->flags = bswap_32(set8->flags);
> -				for (i = 0; i < set8->cnt; i++) {
> -					set8->pairs[i].flags =
> -						bswap_32(set8->pairs[i].flags);
> -				}
> -			}
>  		}
>  
>  		pr_debug("sorting  addr %5lu: cnt %6d [%s]\n",
> @@ -748,9 +735,6 @@ static int symbols_patch(struct object *obj)
>  	if (sets_patch(obj))
>  		return -1;
>  
> -	/* Set type to ensure endian translation occurs. */
> -	obj->efile.idlist->d_type = ELF_T_WORD;
> -
>  	elf_flagdata(obj->efile.idlist, ELF_C_SET, ELF_F_DIRTY);
>  
>  	err = elf_update(obj->efile.elf, ELF_C_WRITE);





[Index of Archives]     [Linux Wireless]     [Linux Kernel]     [ATH6KL]     [Linux Bluetooth]     [Linux Netdev]     [Kernel Newbies]     [Share Photos]     [IDE]     [Security]     [Git]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Linux ATA RAID]     [Samba]     [Device Mapper]

  Powered by Linux