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

Fangrui Song
2022-05-27T05:52:51Z

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>
---
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;
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;
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