Messages in this thread | | | Date | Fri, 27 May 2022 11:35:30 -0700 | From | Fangrui Song <> | Subject | Re: [PATCH] perf: Fix segbase for ld.lld linked objects |
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On 2022-05-27, Ian Rogers wrote: >On Thu, May 26, 2022 at 11:49 PM Ian Rogers <irogers@google.com> wrote: >> >> On Thu, May 26, 2022 at 10:52 PM Fangrui Song <maskray@google.com> wrote: >> > >> > segbase is the address of .eh_frame_hdr and table_data is segbase plus >> > the header size. find_proc_info computes segbase as `map->start + >> > segbase - map->pgoff` which is wrong when >> > >> > * .eh_frame_hdr and .text are in different PT_LOAD program headers >> > * and their p_vaddr difference does not equal their p_offset difference >> > >> > Since 10.0, ld.lld's default --rosegment -z noseparate-code layout has >> > such R and RX PT_LOAD program headers. >> > >> > ld.lld => fail >> > ld.lld --no-rosegment => ok >> > ld.lld -z separate-code => ok >> > >> > ld.bfd -z separate-code (default for Linux/x86) => ok >> > ld.bfd -z noseparate-code => ok >> > >> > To fix the issue, compute segbase as dso's base address plus >> > PT_GNU_EH_FRAME's p_vaddr. >> > >> > Reported-by: Sebastian Ullrich <sebasti@nullri.ch> >> > Link: https://github.com/ClangBuiltLinux/linux/issues/1646 >> > Signed-off-by: Fangrui Song <maskray@google.com> >> > Cc: Ian Rogers <irogers@google.com> >> >> Thanks Fangrui! I tested static vs not, bfd and lld, perf test and >> some hand tests. I couldn't see anything out of order. The code looks >> good although there could be some ambiguity over the variables ending >> _addr as these are the ELF given addresses rather than the thread's >> virtual memory addresses. I'm also trying to get more comments >> especially on major data structures like dso, but that can be fixed as >> a follow-on. >> >> Tested-by: Ian Rogers <irogers@google.com> >> >> > --- >> > tools/perf/util/dso.h | 2 + >> > tools/perf/util/unwind-libunwind-local.c | 96 ++++++++++++++++-------- >> > 2 files changed, 67 insertions(+), 31 deletions(-) >> > >> > diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h >> > index 3a9fd4d389b5..ec4fc1a9454b 100644 >> > --- a/tools/perf/util/dso.h >> > +++ b/tools/perf/util/dso.h >> > @@ -179,6 +179,7 @@ struct dso { >> > bool loaded; >> > u8 rel; >> > struct build_id bid; >> > + u64 base_addr; > >This is an address in a thread, as one dso can be shared by multiple >threads/processes it seems unsafe to hold it as the dso may be loaded >at a different address in a different thread. For now we can just >recompute the value rather than store it in the dso.
Ack. Thanks for spotting the issue! I did not know that dso can be shared (read the code for the first time), guessing this is why `dso__data_get_fd(dso, ui->machine);` needs a lock.
>> > u64 text_offset; >> > const char *short_name; >> > const char *long_name; >> > @@ -197,6 +198,7 @@ struct dso { >> > u64 file_size; >> > struct list_head open_entry; >> > u64 debug_frame_offset; >> > + u64 eh_frame_hdr_addr; > >This is the elf section address and safe to hold here.
Ack.
Fixed in PATCH v2 https://lore.kernel.org/linux-perf-users/20220527182039.673248-1-maskray@google.com/T/#u
>Thanks, >Ian > >> > u64 eh_frame_hdr_offset; >> > } data; >> > /* bpf prog information */ >> > diff --git a/tools/perf/util/unwind-libunwind-local.c b/tools/perf/util/unwind-libunwind-local.c >> > index 41e29fc7648a..7a0053954e49 100644 >> > --- a/tools/perf/util/unwind-libunwind-local.c >> > +++ b/tools/perf/util/unwind-libunwind-local.c >> > @@ -169,30 +169,55 @@ static int __dw_read_encoded_value(u8 **p, u8 *end, u64 *val, >> > __v; \ >> > }) >> > >> > -static u64 elf_section_offset(int fd, const char *name) >> > +static int elf_section_address_and_offset(int fd, const char *name, u64 *address, u64 *offset) >> > { >> > Elf *elf; >> > GElf_Ehdr ehdr; >> > GElf_Shdr shdr; >> > - u64 offset = 0; >> > + int ret; >> > >> > elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL); >> > if (elf == NULL) >> > - return 0; >> > + return -1; >> > >> > - do { >> > - if (gelf_getehdr(elf, &ehdr) == NULL) >> > - break; >> > + if (gelf_getehdr(elf, &ehdr) == NULL) >> > + goto out_err; >> > >> > - if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL)) >> > - break; >> > - >> > - offset = shdr.sh_offset; >> > - } while (0); >> > + if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL)) >> > + goto out_err; >> > >> > + *address = shdr.sh_addr; >> > + *offset = shdr.sh_offset; >> > + ret = 0; >> > +out_err: >> > elf_end(elf); >> > + return ret; >> > +} >> > + >> > +#ifndef NO_LIBUNWIND_DEBUG_FRAME >> > +static u64 elf_section_offset(int fd, const char *name) >> > +{ >> > + u64 address, offset; >> > + >> > + if (elf_section_address_and_offset(fd, name, &address, &offset)) >> > + return 0; >> > + >> > return offset; >> > } >> > +#endif >> > + >> > +static int elf_base_address(int fd) >> > +{ >> > + Elf *elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL); >> > + GElf_Phdr phdr; >> > + int retval = 0; >> > + >> > + if (gelf_getphdr(elf, 0, &phdr) != NULL) >> > + retval = phdr.p_vaddr & -getpagesize(); >> > + >> > + elf_end(elf); >> > + return retval; >> > +} >> > >> > #ifndef NO_LIBUNWIND_DEBUG_FRAME >> > static int elf_is_exec(int fd, const char *name) >> > @@ -248,8 +273,7 @@ struct eh_frame_hdr { >> > } __packed; >> > >> > static int unwind_spec_ehframe(struct dso *dso, struct machine *machine, >> > - u64 offset, u64 *table_data, u64 *segbase, >> > - u64 *fde_count) >> > + u64 offset, u64 *table_data_offset, u64 *fde_count) >> > { >> > struct eh_frame_hdr hdr; >> > u8 *enc = (u8 *) &hdr.enc; >> > @@ -265,35 +289,45 @@ static int unwind_spec_ehframe(struct dso *dso, struct machine *machine, >> > dw_read_encoded_value(enc, end, hdr.eh_frame_ptr_enc); >> > >> > *fde_count = dw_read_encoded_value(enc, end, hdr.fde_count_enc); >> > - *segbase = offset; >> > - *table_data = (enc - (u8 *) &hdr) + offset; >> > + *table_data_offset = enc - (u8 *) &hdr; >> > return 0; >> > } >> > >> > -static int read_unwind_spec_eh_frame(struct dso *dso, struct machine *machine, >> > +static int read_unwind_spec_eh_frame(struct dso *dso, struct unwind_info *ui, >> > u64 *table_data, u64 *segbase, >> > u64 *fde_count) >> > { >> > - int ret = -EINVAL, fd; >> > - u64 offset = dso->data.eh_frame_hdr_offset; >> > + struct map *map; >> > + int ret, fd; >> > >> > - if (offset == 0) { >> > - fd = dso__data_get_fd(dso, machine); >> > + if (dso->data.eh_frame_hdr_offset == 0) { >> > + u64 addr_min = UINT64_MAX; >> > + >> > + fd = dso__data_get_fd(dso, ui->machine); >> > if (fd < 0) >> > return -EINVAL; >> > >> > /* Check the .eh_frame section for unwinding info */ >> > - offset = elf_section_offset(fd, ".eh_frame_hdr"); >> > - dso->data.eh_frame_hdr_offset = offset; >> > + ret = elf_section_address_and_offset(fd, ".eh_frame_hdr", >> > + &dso->data.eh_frame_hdr_addr, >> > + &dso->data.eh_frame_hdr_offset); >> > + maps__for_each_entry(ui->thread->maps, map) { >> > + if (map->dso == dso && map->start < addr_min) >> > + addr_min = map->start; >> > + } >> > + dso->base_addr = addr_min - elf_base_address(fd); >> > dso__data_put_fd(dso); >> > + if (ret || dso->data.eh_frame_hdr_offset == 0) >> > + return -EINVAL; >> > } >> > >> > - if (offset) >> > - ret = unwind_spec_ehframe(dso, machine, offset, >> > - table_data, segbase, >> > - fde_count); >> > - >> > - return ret; >> > + *segbase = dso->base_addr + dso->data.eh_frame_hdr_addr; >> > + ret = unwind_spec_ehframe(dso, ui->machine, dso->data.eh_frame_hdr_offset, >> > + table_data, fde_count); >> > + if (ret) >> > + return ret; >> > + *table_data += *segbase; >> > + return 0; >> > } >> > >> > #ifndef NO_LIBUNWIND_DEBUG_FRAME >> > @@ -388,14 +422,14 @@ find_proc_info(unw_addr_space_t as, unw_word_t ip, unw_proc_info_t *pi, >> > pr_debug("unwind: find_proc_info dso %s\n", map->dso->name); >> > >> > /* Check the .eh_frame section for unwinding info */ >> > - if (!read_unwind_spec_eh_frame(map->dso, ui->machine, >> > + if (!read_unwind_spec_eh_frame(map->dso, ui, >> > &table_data, &segbase, &fde_count)) { >> > memset(&di, 0, sizeof(di)); >> > di.format = UNW_INFO_FORMAT_REMOTE_TABLE; >> > di.start_ip = map->start; >> > di.end_ip = map->end; >> > - di.u.rti.segbase = map->start + segbase - map->pgoff; >> > - di.u.rti.table_data = map->start + table_data - map->pgoff; >> > + di.u.rti.segbase = segbase; >> > + di.u.rti.table_data = table_data; >> > di.u.rti.table_len = fde_count * sizeof(struct table_entry) >> > / sizeof(unw_word_t); >> > ret = dwarf_search_unwind_table(as, ip, &di, pi, >> > -- >> > 2.36.1.124.g0e6072fb45-goog >> >
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