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Support separate debug symbol files (as in GDB) #253

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Support separate debug symbol files (as in GDB) #253

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19 changes: 14 additions & 5 deletions backward.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -261,6 +261,7 @@
#else
#include <dlfcn.h>
#endif
#include <libgen.h>
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I expect libgen.h header to exist when libdwarf is available, but that can be not true.

#endif

#if (BACKWARD_HAS_BACKTRACE == 1) || (BACKWARD_HAS_BACKTRACE_SYMBOL == 1)
Expand Down Expand Up @@ -2152,7 +2153,7 @@ class TraceResolverLinuxImpl<trace_resolver_tag::libdwarf>
}

trace.object_filename = resolve_exec_path(symbol_info);
dwarf_fileobject &fobj = load_object_with_dwarf(symbol_info.dli_fname);
dwarf_fileobject &fobj = load_object_with_dwarf(symbol_info.dli_fname, trace.object_filename);
if (!fobj.dwarf_handle) {
return trace; // sad, we couldn't load the object :(
}
Expand Down Expand Up @@ -2312,7 +2313,7 @@ class TraceResolverLinuxImpl<trace_resolver_tag::libdwarf>
return strcmp(a, b) == 0;
}

dwarf_fileobject &load_object_with_dwarf(const std::string &filename_object) {
dwarf_fileobject &load_object_with_dwarf(const std::string &filename_object, const std::string &object_filename) {

if (!_dwarf_loaded) {
// Set the ELF library operating version
Expand Down Expand Up @@ -2431,17 +2432,25 @@ class TraceResolverLinuxImpl<trace_resolver_tag::libdwarf>
ELF_GET_DATA(32)
} else if (e_ident[EI_CLASS] == ELFCLASS64) {
// libelf might have been built without 64 bit support
#if __LIBELF64
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@alexezeder alexezeder Jan 17, 2022 •
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This is a bit unrelated, but libelf installed on my PC (it's Arch Linux, but it's true also for Ubuntu 20.04 at least) does not have this definition, so the entire branch is omitted, so no 64-bit data, so no debuglink info. Probably we can work it around in a better way, but this line really stops me from having nice source locations.

ELF_GET_DATA(64)
#endif
}

if (!debuglink.empty()) {
// We have a debuglink section! Open an elf instance on that
// file instead. If we can't open the file, then return
// the elf handle we had already opened.
dwarf_file_t debuglink_file;
debuglink_file.reset(open(debuglink.c_str(), O_RDONLY));
{
char* object_filename_copy = strdup(object_filename.c_str());
const std::string directory_path = std::string(dirname(object_filename_copy));
free(object_filename_copy);
debuglink_file.reset(
open((directory_path + '/' + debuglink).c_str(), O_RDONLY));
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I expect debuglink to have only filename since, according to docs:

The executable contains a debug link that specifies the name of the separate debug info file.

Also, objcopy tool writes only the filename component in the .gnu_debuglink section even if an absolute path is provided:
https://github.com/bminor/binutils-gdb/blob/b4edb38e827974ecdbc4a6712d8b21bf876018a2/bfd/opncls.c#L1707-L1708

if (debuglink_file.get() < 0) {
debuglink_file.reset(open(
(directory_path + "/.debug/" + debuglink).c_str(), O_RDONLY));
}
}
Comment on lines +2443 to +2453
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There are only 2 locations covered since I don't know what global debug directories contain.

if (debuglink_file.get() > 0) {
dwarf_elf_t debuglink_elf;
debuglink_elf.reset(elf_begin(debuglink_file.get(), ELF_C_READ, NULL));
Expand Down