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[libunwind] Add proper support for DWARF unwind on bare metal.

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Right now, ARM EHABI unwind on bare metal expects to find the symbols
__exidx_start and __exidx_end defined, and uses those to locate
the EH tables. However, DWARF unwind on bare metal expects to find
dl_iterate_phdr, which, although possible to provide, is inconvenient
and mildly absurd.

This commit provides feature parity with ARM EHABI unwind by looking
for symbols __eh_frame_start, __eh_frame_end, __eh_frame_hdr_start
and __eh_frame_hdr_end, denoting the start and end of the sections
with corresponding names. As far as I know, there is no de jure or
de facto ABI providing any such names, so I chose the obvious ones.

The .eh_frame_hdr support is optional for maximum flexibility and
possible space savings (e.g. if libunwind is only used to provide
backtraces when a device crashes, providing the .eh_frame_hdr, which
is an index for rapid access to EH tables, would be a waste.)

The support for .eh_frame_hdr/DWARF index in the first place is
conditional on defined(_LIBUNWIND_SUPPORT_DWARF_INDEX), although
right now config.h will always define this macro.

The support for DWARF unwind on bare metal has been validated within
the ARTIQ environment[1].

  [1]: https://m-labs.hk/artiq/

llvm-svn: 321445
This commit is contained in:
whitequark
2017-12-25 17:05:07 +00:00
parent ed8d2bc5c6
commit dcd93505da
1 changed files with 46 additions and 0 deletions
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46
libunwind/src/AddressSpace.hpp
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@@ -83,6 +83,38 @@ namespace libunwind {
}
#endif
#elif defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND) && defined(_LIBUNWIND_IS_BAREMETAL)
// When statically linked on bare-metal, the symbols for the EH table are looked
// up without going through the dynamic loader.
// The following linker script may be used to produce the necessary sections and symbols.
// Unless the --eh-frame-hdr linker option is provided, the section is not generated
// and does not take space in the output file.
//
// .eh_frame :
// {
// __eh_frame_start = .;
// KEEP(*(.eh_frame))
// __eh_frame_end = .;
// }
//
// .eh_frame_hdr :
// {
// KEEP(*(.eh_frame_hdr))
// }
//
// __eh_frame_hdr_start = SIZEOF(.eh_frame_hdr) > 0 ? ADDR(.eh_frame_hdr) : 0;
// __eh_frame_hdr_end = SIZEOF(.eh_frame_hdr) > 0 ? . : 0;
extern char __eh_frame_start;
extern char __eh_frame_end;
#if defined(_LIBUNWIND_SUPPORT_DWARF_INDEX)
extern char __eh_frame_hdr_start;
extern char __eh_frame_hdr_end;
#endif
#elif defined(_LIBUNWIND_ARM_EHABI) && defined(_LIBUNWIND_IS_BAREMETAL)
// When statically linked on bare-metal, the symbols for the EH table are looked
@@ -348,6 +380,20 @@ inline bool LocalAddressSpace::findUnwindSections(pint_t targetAddr,
info.compact_unwind_section_length = dyldInfo.compact_unwind_section_length;
return true;
}
#elif defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND) && defined(_LIBUNWIND_IS_BAREMETAL)
// Bare metal is statically linked, so no need to ask the dynamic loader
info.dwarf_section_length = (uintptr_t)(&__eh_frame_end - &__eh_frame_start);
info.dwarf_section = (uintptr_t)(&__eh_frame_start);
_LIBUNWIND_TRACE_UNWINDING("findUnwindSections: section %X length %x",
info.dwarf_section, info.dwarf_section_length);
#if defined(_LIBUNWIND_SUPPORT_DWARF_INDEX)
info.dwarf_index_section = (uintptr_t)(&__eh_frame_hdr_start);
info.dwarf_index_section_length = (uintptr_t)(&__eh_frame_hdr_end - &__eh_frame_hdr_start);
_LIBUNWIND_TRACE_UNWINDING("findUnwindSections: index section %X length %x",
info.dwarf_index_section, info.dwarf_index_section_length);
#endif
if (info.dwarf_section_length)
return true;
#elif defined(_LIBUNWIND_ARM_EHABI) && defined(_LIBUNWIND_IS_BAREMETAL)
// Bare metal is statically linked, so no need to ask the dynamic loader
info.arm_section = (uintptr_t)(&__exidx_start);