On Fri, Nov 22, 2024 at 01:44:31PM -0800, Eduard Zingerman wrote:
> btf_encoder__tag_kfuncs() reads .BTF_ids section to identify a set of
> kfuncs present in the ELF file being processed.
> This section consists of:
> - arrays of uint32_t elements;
> - arrays of records with the following structure:
> struct btf_id_and_flag {
> uint32_t id;
> uint32_t flags;
> };
>
> When endianness of a binary operated by pahole differs from the host
> system's endianness, these fields require byte-swapping before use.
> Currently, this byte-swapping does not occur, resulting in kfuncs not
> being marked with declaration tags.
>
> This commit resolves the issue by introducing an endianness conversion
> step for the .BTF_ids section data before the main processing stage.
> Since the ELF file is opened in O_RDONLY mode, gelf_xlatetom()
> cannot be used for endianness conversion.
> Instead, a new type is introduced:
>
> struct local_elf_data {
> void *d_buf;
> size_t d_size;
> int64_t d_off;
> bool owns_buf;
> };
>
> This structure is populated from the Elf_Data object representing
> the .BTF_ids section. When byte-swapping is required, a local copy
> of d_buf is created.
>
> Cc: Alan Maguire <alan.maguire@xxxxxxxxxx>
> Cc: Daniel Xu <dxu@xxxxxxxxx>
> Cc: Jiri Olsa <olsajiri@xxxxxxxxx>
> Cc: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> Cc: Vadim Fedorenko <vadfed@xxxxxxxx>
> Fixes: 72e88f29c6f7 ("pahole: Inject kfunc decl tags into BTF")
> Reviewed-by: Vadim Fedorenko <vadim.fedorenko@xxxxxxxxx>
> Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
thanks,
jirka
> ---
> btf_encoder.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 59 insertions(+), 6 deletions(-)
>
> diff --git a/btf_encoder.c b/btf_encoder.c
> index e1adddf..06d4a61 100644
> --- a/btf_encoder.c
> +++ b/btf_encoder.c
> @@ -33,6 +33,7 @@
> #include <stdint.h>
> #include <search.h> /* for tsearch(), tfind() and tdestroy() */
> #include <pthread.h>
> +#include <byteswap.h>
>
> #define BTF_IDS_SECTION ".BTF_ids"
> #define BTF_ID_FUNC_PFX "__BTF_ID__func__"
> @@ -145,6 +146,14 @@ struct btf_kfunc_set_range {
> uint64_t end;
> };
>
> +/* Like Elf_Data, but when there is a need to change the data read from ELF */
> +struct local_elf_data {
> + void *d_buf;
> + size_t d_size;
> + int64_t d_off;
> + bool owns_buf;
> +};
> +
> static LIST_HEAD(encoders);
> static pthread_mutex_t encoders__lock = PTHREAD_MUTEX_INITIALIZER;
>
> @@ -1681,7 +1690,8 @@ out:
> }
>
> /* Returns if `sym` points to a kfunc set */
> -static int is_sym_kfunc_set(GElf_Sym *sym, const char *name, Elf_Data *idlist, size_t idlist_addr)
> +static int is_sym_kfunc_set(GElf_Sym *sym, const char *name, struct local_elf_data *idlist,
> + size_t idlist_addr)
> {
> void *ptr = idlist->d_buf;
> struct btf_id_set8 *set;
> @@ -1847,13 +1857,52 @@ static int btf_encoder__tag_kfunc(struct btf_encoder *encoder, struct gobuffer *
> return 0;
> }
>
> +/* If byte order of 'elf' differs from current byte order, convert the data->d_buf.
> + * ELF file is opened in a readonly mode, so data->d_buf cannot be modified in place.
> + * Instead, allocate a new buffer if modification is necessary.
> + */
> +static int convert_idlist_endianness(Elf *elf, Elf_Data *src, struct local_elf_data *dst)
> +{
> + int byteorder, i;
> + char *elf_ident;
> + uint32_t *tmp;
> +
> + dst->d_size = src->d_size;
> + dst->d_off = src->d_off;
> + elf_ident = elf_getident(elf, NULL);
> + if (elf_ident == NULL) {
> + fprintf(stderr, "Cannot get ELF identification from header\n");
> + return -EINVAL;
> + }
> + byteorder = elf_ident[EI_DATA];
> + if ((BYTE_ORDER == LITTLE_ENDIAN && byteorder == ELFDATA2LSB)
> + || (BYTE_ORDER == BIG_ENDIAN && byteorder == ELFDATA2MSB)) {
> + dst->d_buf = src->d_buf;
> + dst->owns_buf = false;
> + return 0;
> + }
> + tmp = malloc(src->d_size);
> + if (tmp == NULL) {
> + fprintf(stderr, "Cannot allocate %lu bytes of memory\n", src->d_size);
> + return -ENOMEM;
> + }
> + memcpy(tmp, src->d_buf, src->d_size);
> + dst->d_buf = tmp;
> + dst->owns_buf = true;
> +
> + /* .BTF_ids sections consist of u32 objects */
> + for (i = 0; i < dst->d_size / sizeof(uint32_t); i++)
> + tmp[i] = bswap_32(tmp[i]);
> + return 0;
> +}
> +
> static int btf_encoder__tag_kfuncs(struct btf_encoder *encoder)
> {
> const char *filename = encoder->source_filename;
> struct gobuffer btf_kfunc_ranges = {};
> + struct local_elf_data idlist = {};
> struct gobuffer btf_funcs = {};
> Elf_Data *symbols = NULL;
> - Elf_Data *idlist = NULL;
> Elf_Scn *symscn = NULL;
> int symbols_shndx = -1;
> size_t idlist_addr = 0;
> @@ -1918,7 +1967,9 @@ static int btf_encoder__tag_kfuncs(struct btf_encoder *encoder)
> } else if (!strcmp(secname, BTF_IDS_SECTION)) {
> idlist_shndx = i;
> idlist_addr = shdr.sh_addr;
> - idlist = data;
> + err = convert_idlist_endianness(elf, data, &idlist);
> + if (err < 0)
> + goto out;
> }
> }
>
> @@ -1960,7 +2011,7 @@ static int btf_encoder__tag_kfuncs(struct btf_encoder *encoder)
> continue;
>
> name = elf_strptr(elf, strtabidx, sym.st_name);
> - if (!is_sym_kfunc_set(&sym, name, idlist, idlist_addr))
> + if (!is_sym_kfunc_set(&sym, name, &idlist, idlist_addr))
> continue;
>
> range.start = sym.st_value;
> @@ -2003,13 +2054,13 @@ static int btf_encoder__tag_kfuncs(struct btf_encoder *encoder)
> if (ranges[j].start <= addr && addr < ranges[j].end) {
> found = true;
> off = addr - idlist_addr;
> - if (off < 0 || off + sizeof(*pair) > idlist->d_size) {
> + if (off < 0 || off + sizeof(*pair) > idlist.d_size) {
> fprintf(stderr, "%s: kfunc '%s' offset outside section '%s'\n",
> __func__, func, BTF_IDS_SECTION);
> free(func);
> goto out;
> }
> - pair = idlist->d_buf + off;
> + pair = idlist.d_buf + off;
> break;
> }
> }
> @@ -2031,6 +2082,8 @@ static int btf_encoder__tag_kfuncs(struct btf_encoder *encoder)
> out:
> __gobuffer__delete(&btf_funcs);
> __gobuffer__delete(&btf_kfunc_ranges);
> + if (idlist.owns_buf)
> + free(idlist.d_buf);
> if (elf)
> elf_end(elf);
> if (fd != -1)
> --
> 2.47.0
>
