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

Ian Rogers
2022-05-28T14:49:32Z

On Sat, May 28, 2022 at 5:06 AM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> 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?

libdw also is broken with lld but not bfd. When I switch on verbose
output (-vv) with perf report I see:

overlapping maps:
202000-203000 0 a.out
202000-203000 0 a.out

and:

unwind: failed with 'address range overlaps an existing module'

The sense I get from this is libdw is rejecting the file before
anything we can do in perf.

Thanks,
Ian

> 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
> >