From 6e9dcfb906f734b1e5b44d4ca2fac0a87bbac777 Mon Sep 17 00:00:00 2001 From: Ilya Leoshkevich Date: Wed, 23 Oct 2024 22:27:41 +0200 Subject: [PATCH] linux-user: Fix GDB complaining about system-supplied DSO string table index MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit When debugging qemu-user processes using gdbstub, the following warning appears every time: warning: BFD: warning: system-supplied DSO at 0x7f8253cc3000 has a corrupt string table index The reason is that QEMU does not map the VDSO's section headers. The VDSO's ELF header's e_shoff points to zeros, which GDB fails to parse. The difference with the kernel's VDSO is that the latter is mapped as a blob, ignoring program headers - which also don't cover the section table. QEMU, on the other hand, loads it as an ELF file. There appears to be no way to place section headers inside a section, and, therefore, no way to refer to them from a linker script. Also, ld hardcodes section headers to be non-loadable, see _bfd_elf_assign_file_positions_for_non_load(). In theory ld could be enhanced by implementing an "SHDRS" keyword in addition to the existing "FILEHDR" and "PHDRS". There are multiple ways to resolve the issue: - Copy VDSO as a blob in load_elf_vdso(). This would require creating specialized loader logic, that duplicates parts of load_elf_image(). - Fix up VDSO's PHDR size in load_elf_vdso(). This would require either duplicating the parsing logic, or adding an ugly parameter to load_elf_image(). - Fix up VDSO's PHDR size in gen-vdso. This is the simplest solution, so do it. There are two tricky parts: - Byte-swaps need to be done either on local copies, or in-place and then reverted in the end. To preserve the existing code structure, do the former for Sym and Dyn, and the latter for Ehdr, Phdr, and Shdr. - There must be no .bss, which is already the case - but having an explicit check is helpful to ensure correctness. To verify this change, I diffed the on-disk and the loaded VDSOs; the result does not show anything unusual, except for what seems to be an existing oversight (which should probably be fixed separately): │ Symbol table '.dynsym' contains 8 entries: │ Num: Value Size Type Bind Vis Ndx Name │ - 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND │ - 6: 0000000000000000 0 OBJECT GLOBAL DEFAULT ABS LINUX_2.6.29 │ + 0: 00007f61075bf000 0 NOTYPE LOCAL DEFAULT UND │ + 6: 00007f61075bf000 0 OBJECT GLOBAL DEFAULT ABS LINUX_2.6.29 Fixes: 2fa536d10797 ("linux-user: Add gen-vdso tool") Signed-off-by: Ilya Leoshkevich Reviewed-by: Richard Henderson Message-ID: <20241023202850.55211-1-iii@linux.ibm.com> Signed-off-by: Richard Henderson --- linux-user/gen-vdso-elfn.c.inc | 98 +++++++++++++++++++++++----------- linux-user/gen-vdso.c | 40 +++++++------- 2 files changed, 87 insertions(+), 51 deletions(-) diff --git a/linux-user/gen-vdso-elfn.c.inc b/linux-user/gen-vdso-elfn.c.inc index 95856eb839..b47019e136 100644 --- a/linux-user/gen-vdso-elfn.c.inc +++ b/linux-user/gen-vdso-elfn.c.inc @@ -68,28 +68,45 @@ static void elfN(search_symtab)(ElfN(Shdr) *shdr, unsigned sym_idx, void *buf, bool need_bswap) { unsigned str_idx = shdr[sym_idx].sh_link; - ElfN(Sym) *sym = buf + shdr[sym_idx].sh_offset; - unsigned sym_n = shdr[sym_idx].sh_size / sizeof(*sym); + ElfN(Sym) *target_sym = buf + shdr[sym_idx].sh_offset; + unsigned sym_n = shdr[sym_idx].sh_size / sizeof(*target_sym); const char *str = buf + shdr[str_idx].sh_offset; for (unsigned i = 0; i < sym_n; ++i) { const char *name; + ElfN(Sym) sym; + memcpy(&sym, &target_sym[i], sizeof(sym)); if (need_bswap) { - elfN(bswap_sym)(sym + i); + elfN(bswap_sym)(&sym); } - name = str + sym[i].st_name; + name = str + sym.st_name; if (sigreturn_sym && strcmp(sigreturn_sym, name) == 0) { - sigreturn_addr = sym[i].st_value; + sigreturn_addr = sym.st_value; } if (rt_sigreturn_sym && strcmp(rt_sigreturn_sym, name) == 0) { - rt_sigreturn_addr = sym[i].st_value; + rt_sigreturn_addr = sym.st_value; } } } -static void elfN(process)(FILE *outf, void *buf, bool need_bswap) +static void elfN(bswap_ps_hdrs)(ElfN(Ehdr) *ehdr) +{ + ElfN(Phdr) *phdr = (void *)ehdr + ehdr->e_phoff; + ElfN(Shdr) *shdr = (void *)ehdr + ehdr->e_shoff; + ElfN(Half) i; + + for (i = 0; i < ehdr->e_phnum; ++i) { + elfN(bswap_phdr)(&phdr[i]); + } + + for (i = 0; i < ehdr->e_shnum; ++i) { + elfN(bswap_shdr)(&shdr[i]); + } +} + +static void elfN(process)(FILE *outf, void *buf, long len, bool need_bswap) { ElfN(Ehdr) *ehdr = buf; ElfN(Phdr) *phdr; @@ -103,24 +120,14 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap) int errors = 0; if (need_bswap) { - elfN(bswap_ehdr)(ehdr); + elfN(bswap_ehdr)(buf); + elfN(bswap_ps_hdrs)(buf); } phnum = ehdr->e_phnum; phdr = buf + ehdr->e_phoff; - if (need_bswap) { - for (unsigned i = 0; i < phnum; ++i) { - elfN(bswap_phdr)(phdr + i); - } - } - shnum = ehdr->e_shnum; shdr = buf + ehdr->e_shoff; - if (need_bswap) { - for (unsigned i = 0; i < shnum; ++i) { - elfN(bswap_shdr)(shdr + i); - } - } for (unsigned i = 0; i < shnum; ++i) { switch (shdr[i].sh_type) { case SHT_SYMTAB: @@ -154,7 +161,24 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap) fprintf(stderr, "LOAD segment not loaded at address 0\n"); errors++; } - first_segsz = phdr[i].p_filesz; + /* + * Extend the program header to cover the entire VDSO, so that + * load_elf_vdso() loads everything, including section headers. + * + * Require that there is no .bss, since it would break this + * approach. + */ + if (phdr[i].p_filesz != phdr[i].p_memsz) { + fprintf(stderr, "LOAD segment's filesz and memsz differ\n"); + errors++; + } + if (phdr[i].p_filesz > len) { + fprintf(stderr, "LOAD segment is larger than the whole VDSO\n"); + errors++; + } + phdr[i].p_filesz = len; + phdr[i].p_memsz = len; + first_segsz = len; if (first_segsz < ehdr->e_phoff + phnum * sizeof(*phdr)) { fprintf(stderr, "LOAD segment does not cover PHDRs\n"); errors++; @@ -197,17 +221,24 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap) output_reloc(outf, buf, &phdr[i].p_paddr); } + /* Relocate the section headers. */ + for (unsigned i = 0; i < shnum; ++i) { + output_reloc(outf, buf, &shdr[i].sh_addr); + } + /* Relocate the DYNAMIC entries. */ if (dynamic_addr) { - ElfN(Dyn) *dyn = buf + dynamic_ofs; - __typeof(dyn->d_tag) tag; + ElfN(Dyn) *target_dyn = buf + dynamic_ofs; + __typeof(((ElfN(Dyn) *)target_dyn)->d_tag) tag; do { + ElfN(Dyn) dyn; + memcpy(&dyn, target_dyn, sizeof(dyn)); if (need_bswap) { - elfN(bswap_dyn)(dyn); + elfN(bswap_dyn)(&dyn); } - tag = dyn->d_tag; + tag = dyn.d_tag; switch (tag) { case DT_HASH: @@ -218,7 +249,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap) case DT_PLTGOT: case DT_ADDRRNGLO ... DT_ADDRRNGHI: /* These entries store an address in the entry. */ - output_reloc(outf, buf, &dyn->d_un.d_val); + output_reloc(outf, buf, &target_dyn->d_un.d_val); break; case DT_NULL: @@ -235,7 +266,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap) break; case DT_SYMENT: - if (dyn->d_un.d_val != sizeof(ElfN(Sym))) { + if (dyn.d_un.d_val != sizeof(ElfN(Sym))) { fprintf(stderr, "VDSO has incorrect dynamic symbol size\n"); errors++; } @@ -251,7 +282,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap) * ??? The RISC-V toolchain will emit these even when there * are no relocations. Validate zeros. */ - if (dyn->d_un.d_val != 0) { + if (dyn.d_un.d_val != 0) { fprintf(stderr, "VDSO has dynamic relocations\n"); errors++; } @@ -287,7 +318,7 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap) errors++; break; } - dyn++; + target_dyn++; } while (tag != DT_NULL); if (errors) { exit(EXIT_FAILURE); @@ -296,11 +327,11 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap) /* Relocate the dynamic symbol table. */ if (dynsym_idx) { - ElfN(Sym) *sym = buf + shdr[dynsym_idx].sh_offset; - unsigned sym_n = shdr[dynsym_idx].sh_size / sizeof(*sym); + ElfN(Sym) *target_sym = buf + shdr[dynsym_idx].sh_offset; + unsigned sym_n = shdr[dynsym_idx].sh_size / sizeof(*target_sym); for (unsigned i = 0; i < sym_n; ++i) { - output_reloc(outf, buf, &sym[i].st_value); + output_reloc(outf, buf, &target_sym[i].st_value); } } @@ -311,4 +342,9 @@ static void elfN(process)(FILE *outf, void *buf, bool need_bswap) if (symtab_idx) { elfN(search_symtab)(shdr, symtab_idx, buf, need_bswap); } + + if (need_bswap) { + elfN(bswap_ps_hdrs)(buf); + elfN(bswap_ehdr)(buf); + } } diff --git a/linux-user/gen-vdso.c b/linux-user/gen-vdso.c index 31e333be80..721f38d5a3 100644 --- a/linux-user/gen-vdso.c +++ b/linux-user/gen-vdso.c @@ -131,23 +131,6 @@ int main(int argc, char **argv) } fclose(inf); - /* - * Write out the vdso image now, before we make local changes. - */ - - fprintf(outf, - "/* Automatically generated from linux-user/gen-vdso.c. */\n" - "\n" - "static const uint8_t %s_image[] = {", - prefix); - for (long i = 0; i < total_len; ++i) { - if (i % 12 == 0) { - fputs("\n ", outf); - } - fprintf(outf, " 0x%02x,", buf[i]); - } - fprintf(outf, "\n};\n\n"); - /* * Identify which elf flavor we're processing. * The first 16 bytes of the file are e_ident. @@ -179,14 +162,17 @@ int main(int argc, char **argv) * Output relocation addresses as we go. */ - fprintf(outf, "static const unsigned %s_relocs[] = {\n", prefix); + fprintf(outf, + "/* Automatically generated by linux-user/gen-vdso.c. */\n" + "\n" + "static const unsigned %s_relocs[] = {\n", prefix); switch (buf[EI_CLASS]) { case ELFCLASS32: - elf32_process(outf, buf, need_bswap); + elf32_process(outf, buf, total_len, need_bswap); break; case ELFCLASS64: - elf64_process(outf, buf, need_bswap); + elf64_process(outf, buf, total_len, need_bswap); break; default: fprintf(stderr, "%s: invalid elf EI_CLASS (%u)\n", @@ -196,6 +182,20 @@ int main(int argc, char **argv) fprintf(outf, "};\n\n"); /* end vdso_relocs. */ + /* + * Write out the vdso image now, after we made local changes. + */ + fprintf(outf, + "static const uint8_t %s_image[] = {", + prefix); + for (long i = 0; i < total_len; ++i) { + if (i % 12 == 0) { + fputs("\n ", outf); + } + fprintf(outf, " 0x%02x,", buf[i]); + } + fprintf(outf, "\n};\n\n"); + fprintf(outf, "static const VdsoImageInfo %s_image_info = {\n", prefix); fprintf(outf, " .image = %s_image,\n", prefix); fprintf(outf, " .relocs = %s_relocs,\n", prefix);