Re: [PATCH v2] perf: Fix segbase for ld.lld linked objects

Jiri Olsa
2022-05-28T12:07:07Z

On Fri, May 27, 2022 at 11:20:39AM -0700, Fangrui Song 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 (default) => perf report fails to unwind `perf record
> --call-graph dwarf` recorded data
> ld.lld --no-rosegment => ok (trivial, no R PT_LOAD)
> ld.lld -z separate-code => ok but by luck: there are two PT_LOAD but
> their p_vaddr difference equals p_offset difference
>
> ld.bfd -z noseparate-code => ok (trivial, no R PT_LOAD)
> ld.bfd -z separate-code (default for Linux/x86) => ok but by luck:
> there are two PT_LOAD but their p_vaddr difference equals p_offset
> difference
>
> To fix the issue, compute segbase as dso's base address plus
> PT_GNU_EH_FRAME's p_vaddr. The base address is computed by iterating
> over all dso-associated maps and then subtract the first PT_LOAD p_vaddr
> (the minimum guaranteed by generic ABI) from the minimum address.
>
> In libunwind, find_proc_info transitively called by unw_step is cached,
> so the iteration overhead is acceptable.
>
> 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>

nice, is this a problem for libdw dwarf unwind as well?

you should be able to build it with:

$ make NO_LIBUNWIND=1

thanks,
jirka

>
> --
> Changes from v1:
> * Fix elf_base_address to use the first PT_LOAD
> * Use dso::elf_base_addr which is a constant even if the dso is loaded into multiple processes
> ---
> tools/perf/util/dso.h | 2 +
> tools/perf/util/unwind-libunwind-local.c | 107 ++++++++++++++++-------
> 2 files changed, 78 insertions(+), 31 deletions(-)
>
> diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
> index 3a9fd4d389b5..97047a11282b 100644
> --- a/tools/perf/util/dso.h
> +++ b/tools/perf/util/dso.h
> @@ -196,7 +196,9 @@ struct dso {
> u32 status_seen;
> u64 file_size;
> struct list_head open_entry;
> + u64 elf_base_addr;
> u64 debug_frame_offset;
> + u64 eh_frame_hdr_addr;
> 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..37622699c91a 100644
> --- a/tools/perf/util/unwind-libunwind-local.c
> +++ b/tools/perf/util/unwind-libunwind-local.c
> @@ -169,30 +169,64 @@ 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 u64 elf_base_address(int fd)
> +{
> + Elf *elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
> + GElf_Phdr phdr;
> + u64 retval = 0;
> + size_t i, phdrnum = 0;
> +
> + if (elf == NULL)
> + return 0;
> + (void)elf_getphdrnum(elf, &phdrnum);
> + /* PT_LOAD segments are sorted by p_vaddr, so the first has the minimum p_vaddr. */
> + for (i = 0; i < phdrnum; i++) {
> + if (gelf_getphdr(elf, i, &phdr) && phdr.p_type == PT_LOAD) {
> + retval = phdr.p_vaddr & -getpagesize();
> + break;
> + }
> + }
> +
> + elf_end(elf);
> + return retval;
> +}
>
> #ifndef NO_LIBUNWIND_DEBUG_FRAME
> static int elf_is_exec(int fd, const char *name)
> @@ -248,8 +282,7 @@ 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 +298,47 @@ 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;
> + u64 base_addr = UINT64_MAX;
> + int ret, fd;
>
> - if (offset == 0) {
> - fd = dso__data_get_fd(dso, machine);
> + if (dso->data.eh_frame_hdr_offset == 0) {
> + 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);
> + dso->data.elf_base_addr = 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;
> + maps__for_each_entry(ui->thread->maps, map) {
> + if (map->dso == dso && map->start < base_addr)
> + base_addr = map->start;
> + }
> + base_addr -= dso->data.elf_base_addr;
> + /* Address of .eh_frame_hdr */
> + *segbase = 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;
> + /* binary_search_table offset plus .eh_frame_hdr address */
> + *table_data += *segbase;
> + return 0;
> }
>
> #ifndef NO_LIBUNWIND_DEBUG_FRAME
> @@ -388,14 +433,14 @@ 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
>