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linux/tools/perf/util/genelf.c
Eric Biggers f136fc491b perf genelf: Switch from SHA-1 to BLAKE2s for build ID generation 
Recent patches [1] [2] added an implementation of SHA-1 to perf and made
it be used for build ID generation.

I had understood the choice of SHA-1, which is a legacy algorithm, to be
for backwards compatibility.

It turns out, though, that there's no backwards compatibility
requirement here other than the size of the build ID field, which is
fixed at 20 bytes.  Not only did the hash algorithm already change (from
MD5 to SHA-1), but the inputs to the hash changed too: from 'load_addr
|| code' to just 'code', and now again to 'code || symtab || strsym'
[3].  Different linkers generate different build IDs, with the LLVM
linker using BLAKE3 hashes for example [4].

Therefore, we might as well switch to a more modern algorithm.  Let's go
with BLAKE2s.  It's faster than SHA-1, isn't cryptographically broken,
is easier to implement than BLAKE3, and the kernel's implementation in
lib/crypto/blake2s.c is easily borrowed.  It also natively supports
variable-length hashes, so it can directly produce the needed 20 bytes.

Also make the following additional improvements:

- Hash the three inputs incrementally, so they don't all have to be
  concatenated into one buffer.

- Add tag/length prefixes to each of the three inputs, so that distinct
  input tuples reliably result in distinct hashes.

[1] https://lore.kernel.org/linux-perf-users/20250521225307.743726-1-yuzhuo@google.com/
[2] https://lore.kernel.org/linux-perf-users/20250625202311.23244-1-ebiggers@kernel.org/
[3] https://lore.kernel.org/linux-perf-users/20251125080748.461014-1-namhyung@kernel.org/
[4] d3e5b6f753

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Tested-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Fangrui Song <maskray@sourceware.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Pablo Galindo <pablogsal@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
2026-01-13 17:03:28 -03:00

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// SPDX-License-Identifier: GPL-2.0-only
/*
* genelf.c
* Copyright (C) 2014, Google, Inc
*
* Contributed by:
* Stephane Eranian <eranian@gmail.com>
*/
#include <sys/types.h>
#include <stddef.h>
#include <libelf.h>
#include <string.h>
#include <stdlib.h>
#include <inttypes.h>
#include <err.h>
#ifdef HAVE_LIBDW_SUPPORT
#include <dwarf.h>
#endif
#include "blake2s.h"
#include "genelf.h"
#include "../util/jitdump.h"
#include <linux/compiler.h>
#ifndef NT_GNU_BUILD_ID
#define NT_GNU_BUILD_ID 3
#endif
typedef struct {
unsigned int namesz; /* Size of entry's owner string */
unsigned int descsz; /* Size of the note descriptor */
unsigned int type; /* Interpretation of the descriptor */
char name[0]; /* Start of the name+desc data */
} Elf_Note;
static char shd_string_table[] = {
0,
'.', 't', 'e', 'x', 't', 0, /* 1 */
'.', 's', 'h', 's', 't', 'r', 't', 'a', 'b', 0, /* 7 */
'.', 's', 'y', 'm', 't', 'a', 'b', 0, /* 17 */
'.', 's', 't', 'r', 't', 'a', 'b', 0, /* 25 */
'.', 'n', 'o', 't', 'e', '.', 'g', 'n', 'u', '.', 'b', 'u', 'i', 'l', 'd', '-', 'i', 'd', 0, /* 33 */
'.', 'd', 'e', 'b', 'u', 'g', '_', 'l', 'i', 'n', 'e', 0, /* 52 */
'.', 'd', 'e', 'b', 'u', 'g', '_', 'i', 'n', 'f', 'o', 0, /* 64 */
'.', 'd', 'e', 'b', 'u', 'g', '_', 'a', 'b', 'b', 'r', 'e', 'v', 0, /* 76 */
'.', 'e', 'h', '_', 'f', 'r', 'a', 'm', 'e', '_', 'h', 'd', 'r', 0, /* 90 */
'.', 'e', 'h', '_', 'f', 'r', 'a', 'm', 'e', 0, /* 104 */
};
static struct buildid_note {
Elf_Note desc; /* descsz: size of build-id, must be multiple of 4 */
char name[4]; /* GNU\0 */
u8 build_id[20];
} bnote;
static Elf_Sym symtab[]={
/* symbol 0 MUST be the undefined symbol */
{ .st_name = 0, /* index in sym_string table */
.st_info = ELF_ST_TYPE(STT_NOTYPE),
.st_shndx = 0, /* for now */
.st_value = 0x0,
.st_other = ELF_ST_VIS(STV_DEFAULT),
.st_size = 0,
},
{ .st_name = 1, /* index in sym_string table */
.st_info = ELF_ST_BIND(STB_LOCAL) | ELF_ST_TYPE(STT_FUNC),
.st_shndx = 1,
.st_value = 0, /* for now */
.st_other = ELF_ST_VIS(STV_DEFAULT),
.st_size = 0, /* for now */
}
};
static int
jit_add_eh_frame_info(Elf *e, void* unwinding, uint64_t unwinding_header_size,
uint64_t unwinding_size, uint64_t base_offset)
{
Elf_Data *d;
Elf_Scn *scn;
Elf_Shdr *shdr;
uint64_t unwinding_table_size = unwinding_size - unwinding_header_size;
/*
* setup eh_frame section
*/
scn = elf_newscn(e);
if (!scn) {
warnx("cannot create section");
return -1;
}
d = elf_newdata(scn);
if (!d) {
warnx("cannot get new data");
return -1;
}
d->d_align = 8;
d->d_off = 0LL;
d->d_buf = unwinding;
d->d_type = ELF_T_BYTE;
d->d_size = unwinding_table_size;
d->d_version = EV_CURRENT;
shdr = elf_getshdr(scn);
if (!shdr) {
warnx("cannot get section header");
return -1;
}
shdr->sh_name = 104;
shdr->sh_type = SHT_PROGBITS;
shdr->sh_addr = base_offset;
shdr->sh_flags = SHF_ALLOC;
shdr->sh_entsize = 0;
/*
* setup eh_frame_hdr section
*/
scn = elf_newscn(e);
if (!scn) {
warnx("cannot create section");
return -1;
}
d = elf_newdata(scn);
if (!d) {
warnx("cannot get new data");
return -1;
}
d->d_align = 4;
d->d_off = 0LL;
d->d_buf = unwinding + unwinding_table_size;
d->d_type = ELF_T_BYTE;
d->d_size = unwinding_header_size;
d->d_version = EV_CURRENT;
shdr = elf_getshdr(scn);
if (!shdr) {
warnx("cannot get section header");
return -1;
}
shdr->sh_name = 90;
shdr->sh_type = SHT_PROGBITS;
shdr->sh_addr = base_offset + unwinding_table_size;
shdr->sh_flags = SHF_ALLOC;
shdr->sh_entsize = 0;
return 0;
}
enum {
TAG_CODE = 0,
TAG_SYMTAB = 1,
TAG_STRSYM = 2,
};
/*
* Update the hash using the given data, also prepending a (tag, len) prefix to
* ensure that distinct input tuples reliably result in distinct hashes.
*/
static void blake2s_update_tagged(struct blake2s_ctx *ctx, int tag,
const void *data, size_t len)
{
u64 prefix = ((u64)tag << 56) | len;
blake2s_update(ctx, (const u8 *)&prefix, sizeof(prefix));
blake2s_update(ctx, data, len);
}
/*
* fd: file descriptor open for writing for the output file
* load_addr: code load address (could be zero)
* sym: function name (for native code - used as the symbol)
* code: the native code
* csize: the code size in bytes
*/
int
jit_write_elf(int fd, uint64_t load_addr __maybe_unused, const char *sym,
const void *code, int csize,
void *debug __maybe_unused, int nr_debug_entries __maybe_unused,
void *unwinding, uint64_t unwinding_header_size, uint64_t unwinding_size)
{
Elf *e;
Elf_Data *d;
Elf_Scn *scn;
Elf_Ehdr *ehdr;
Elf_Phdr *phdr;
Elf_Shdr *shdr;
uint64_t eh_frame_base_offset;
char *strsym = NULL;
struct blake2s_ctx ctx;
int symlen;
int retval = -1;
if (elf_version(EV_CURRENT) == EV_NONE) {
warnx("ELF initialization failed");
return -1;
}
e = elf_begin(fd, ELF_C_WRITE, NULL);
if (!e) {
warnx("elf_begin failed");
goto error;
}
/*
* setup ELF header
*/
ehdr = elf_newehdr(e);
if (!ehdr) {
warnx("cannot get ehdr");
goto error;
}
ehdr->e_ident[EI_DATA] = GEN_ELF_ENDIAN;
ehdr->e_ident[EI_CLASS] = GEN_ELF_CLASS;
ehdr->e_machine = GEN_ELF_ARCH;
ehdr->e_type = ET_DYN;
ehdr->e_entry = GEN_ELF_TEXT_OFFSET;
ehdr->e_version = EV_CURRENT;
ehdr->e_shstrndx= unwinding ? 4 : 2; /* shdr index for section name */
/*
* setup program header
*/
phdr = elf_newphdr(e, 1);
phdr[0].p_type = PT_LOAD;
phdr[0].p_offset = GEN_ELF_TEXT_OFFSET;
phdr[0].p_vaddr = GEN_ELF_TEXT_OFFSET;
phdr[0].p_paddr = GEN_ELF_TEXT_OFFSET;
phdr[0].p_filesz = csize;
phdr[0].p_memsz = csize;
phdr[0].p_flags = PF_X | PF_R;
phdr[0].p_align = 8;
/*
* setup text section
*/
scn = elf_newscn(e);
if (!scn) {
warnx("cannot create section");
goto error;
}
d = elf_newdata(scn);
if (!d) {
warnx("cannot get new data");
goto error;
}
d->d_align = 16;
d->d_off = 0LL;
d->d_buf = (void *)code;
d->d_type = ELF_T_BYTE;
d->d_size = csize;
d->d_version = EV_CURRENT;
shdr = elf_getshdr(scn);
if (!shdr) {
warnx("cannot get section header");
goto error;
}
shdr->sh_name = 1;
shdr->sh_type = SHT_PROGBITS;
shdr->sh_addr = GEN_ELF_TEXT_OFFSET;
shdr->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
shdr->sh_entsize = 0;
blake2s_init(&ctx, sizeof(bnote.build_id));
blake2s_update_tagged(&ctx, TAG_CODE, code, csize);
/*
* Setup .eh_frame_hdr and .eh_frame
*/
if (unwinding) {
eh_frame_base_offset = ALIGN_8(GEN_ELF_TEXT_OFFSET + csize);
retval = jit_add_eh_frame_info(e, unwinding,
unwinding_header_size, unwinding_size,
eh_frame_base_offset);
if (retval)
goto error;
retval = -1;
}
/*
* setup section headers string table
*/
scn = elf_newscn(e);
if (!scn) {
warnx("cannot create section");
goto error;
}
d = elf_newdata(scn);
if (!d) {
warnx("cannot get new data");
goto error;
}
d->d_align = 1;
d->d_off = 0LL;
d->d_buf = shd_string_table;
d->d_type = ELF_T_BYTE;
d->d_size = sizeof(shd_string_table);
d->d_version = EV_CURRENT;
shdr = elf_getshdr(scn);
if (!shdr) {
warnx("cannot get section header");
goto error;
}
shdr->sh_name = 7; /* offset of '.shstrtab' in shd_string_table */
shdr->sh_type = SHT_STRTAB;
shdr->sh_flags = 0;
shdr->sh_entsize = 0;
/*
* setup symtab section
*/
symtab[1].st_size = csize;
symtab[1].st_value = GEN_ELF_TEXT_OFFSET;
scn = elf_newscn(e);
if (!scn) {
warnx("cannot create section");
goto error;
}
d = elf_newdata(scn);
if (!d) {
warnx("cannot get new data");
goto error;
}
d->d_align = 8;
d->d_off = 0LL;
d->d_buf = symtab;
d->d_type = ELF_T_SYM;
d->d_size = sizeof(symtab);
d->d_version = EV_CURRENT;
shdr = elf_getshdr(scn);
if (!shdr) {
warnx("cannot get section header");
goto error;
}
shdr->sh_name = 17; /* offset of '.symtab' in shd_string_table */
shdr->sh_type = SHT_SYMTAB;
shdr->sh_flags = 0;
shdr->sh_entsize = sizeof(Elf_Sym);
shdr->sh_link = unwinding ? 6 : 4; /* index of .strtab section */
blake2s_update_tagged(&ctx, TAG_SYMTAB, symtab, sizeof(symtab));
/*
* setup symbols string table
* 2 = 1 for 0 in 1st entry, 1 for the 0 at end of symbol for 2nd entry
*/
symlen = 2 + strlen(sym);
strsym = calloc(1, symlen);
if (!strsym) {
warnx("cannot allocate strsym");
goto error;
}
strcpy(strsym + 1, sym);
scn = elf_newscn(e);
if (!scn) {
warnx("cannot create section");
goto error;
}
d = elf_newdata(scn);
if (!d) {
warnx("cannot get new data");
goto error;
}
d->d_align = 1;
d->d_off = 0LL;
d->d_buf = strsym;
d->d_type = ELF_T_BYTE;
d->d_size = symlen;
d->d_version = EV_CURRENT;
shdr = elf_getshdr(scn);
if (!shdr) {
warnx("cannot get section header");
goto error;
}
shdr->sh_name = 25; /* offset in shd_string_table */
shdr->sh_type = SHT_STRTAB;
shdr->sh_flags = 0;
shdr->sh_entsize = 0;
blake2s_update_tagged(&ctx, TAG_STRSYM, strsym, symlen);
/*
* setup build-id section
*/
scn = elf_newscn(e);
if (!scn) {
warnx("cannot create section");
goto error;
}
d = elf_newdata(scn);
if (!d) {
warnx("cannot get new data");
goto error;
}
/*
* build-id generation
*/
blake2s_final(&ctx, bnote.build_id);
bnote.desc.namesz = sizeof(bnote.name); /* must include 0 termination */
bnote.desc.descsz = sizeof(bnote.build_id);
bnote.desc.type = NT_GNU_BUILD_ID;
strcpy(bnote.name, "GNU");
d->d_align = 4;
d->d_off = 0LL;
d->d_buf = &bnote;
d->d_type = ELF_T_BYTE;
d->d_size = sizeof(bnote);
d->d_version = EV_CURRENT;
shdr = elf_getshdr(scn);
if (!shdr) {
warnx("cannot get section header");
goto error;
}
shdr->sh_name = 33; /* offset in shd_string_table */
shdr->sh_type = SHT_NOTE;
shdr->sh_addr = 0x0;
shdr->sh_flags = SHF_ALLOC;
shdr->sh_size = sizeof(bnote);
shdr->sh_entsize = 0;
#ifdef HAVE_LIBDW_SUPPORT
if (debug && nr_debug_entries) {
retval = jit_add_debug_info(e, load_addr, debug, nr_debug_entries);
if (retval)
goto error;
} else
#endif
{
if (elf_update(e, ELF_C_WRITE) < 0) {
warnx("elf_update 4 failed");
goto error;
}
}
retval = 0;
error:
(void)elf_end(e);
free(strsym);
return retval;
}
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