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

[Fangrui Song]

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@xxxxxxxxx>
Link: https://github.com/ClangBuiltLinux/linux/issues/1646
Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
Cc: Ian Rogers <irogers@xxxxxxxxxx>

--
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