On 2020-05-31, Fangrui Song wrote:
It is being discussed on llvm-dev
(https://lists.llvm.org/pipermail/llvm-dev/2020-May/141885.html
https://groups.google.com/forum/#!topic/llvm-dev/i0DFx6YSqDA)
what linkers should do regarding relocations referencing dropped functions (due
to section group rules, --gc-sections, /DISCARD/, etc) in .debug_*
As an example:
__attribute__((section(".text.x"))) void f1() { }
__attribute__((section(".text.x"))) void f2() { }
int main() { }
Some .debug_* sections are relocated by R_X86_64_64 referencing undefined
symbols (the STT_SECTION
symbols are collected):
0x00000043: DW_TAG_subprogram [2]
###### relocated by .text.x + 10
DW_AT_low_pc [DW_FORM_addr] (0x0000000000000010 ".text.x")
DW_AT_high_pc [DW_FORM_data4] (0x00000006)
DW_AT_frame_base [DW_FORM_exprloc] (DW_OP_reg6 RBP)
DW_AT_linkage_name [DW_FORM_strp] ( .debug_str[0x0000002c] =
"_Z2f2v")
DW_AT_name [DW_FORM_strp] ( .debug_str[0x00000033] =
"f2")
With ld --gc-sections:
* DW_AT_low_pc [DW_FORM_addr] in .debug_info are resolved to 0 + addend
This can cause overlapping address ranges with normal text sections.
{{overlap}}
* [beginning address offset, ending address offset) in .debug_ranges are
resolved to 1 (ignoring addend).
See bfd/reloc.c (behavior introduced in
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=e4067dbb2a3368dbf908b39c5435c84d51abc9f3
)
[0, 0) cannot be used because it terminates the list entry.
[-1, -1) cannot be used because -1 represents a base address selection entry
which will affect
subsequent address offset pairs.
* .debug_loc address offset pairs have similar problem to .debug_ranges
* In DWARF v5, the abnormal values can be in a separate section .debug_addr
---
To save your time, I have a summary of the discussions. I am eager to know what
you think
of the ideas from binutils/gdb/elfutils's perspective.
* {{reserved_address}} Paul Robinson wants to propose that DWARF v6 reserves a
special address.
All (undef + addend) in .debug_* are resolved to -1.
We have to ignore the addend. With __attribute__((section(".text.x"))),
the address offset pair may be something like [.text.x + 16, .text.x + 24)
I have to resolve the whole (.text.x + 16) to the special value.
(undef + addend) in pre-DWARF v5 .debug_loc and .debug_ranges are resolved to
-2
(0 and -1 cannot be used due to the reasons above).
* Refined formula for a relocated value in a non-SHF_ALLOC section:
if is_defined(sym)
return addr(sym) + addend
if relocated_section is .debug_ranges or .debug_loc
return -2 # addend is intentionally ignored
// Every DWARF v5 section falls here
return -1 {{zero}}
* {{zero}} Can we resolve (undef + addend) to 0?
https://lists.llvm.org/pipermail/llvm-dev/2020-May/141967.html
> while it might not be an issue for ELF, DWARF would want a standard that's
fairly resilient to
> quirky/interesting use cases (admittedly - such platforms could equally want
to make their
> executable code way up in the address space near max or max - 1, etc?).
Question: is address 0 meaningful for code in some binary formats?
* {{overlap}} The current situation (GNU ld, gold, LLD): (undef + addend) in
.debug_* are resolved to addend.
For an address offset pair like [.text + 0, .text + 0x10010), if the ending
address offset is large
enough, it may overlap with a normal text address range (for example [0x10000,
*))
This can cause problems in debuggers. How does gdb solve the problem?
* {{nonalloc}} Linkers resolve (undef + addend) in non-SHF_ALLOC sections to
`addend`. For non-debug sections (open-ended), do we have needs resolving such
values to `base` or `base+addend` where base is customizable?
(https://lists.llvm.org/pipermail/llvm-dev/2020-May/141956.html )
Forgot to mention
* {{compatibility}} Do we need an option if we change the computed value of
(undef + addend) to
-2 (.debug_loc,.debug_ranges)/-1 (other .debug_*)
(or 0 (other .debug_*), but it might not be nice to some binary formats
{{reserved_address}})
https://lists.llvm.org/pipermail/llvm-dev/2020-May/141958.html
> If we end up blessing it as part of the DWARF spec, we probably
> wouldn't want it to be user-configurable for the .debug_ sections, so
> I'd hesitate to add that configurability to the linker lest we have to
> revoke it to conform to DWARF (breaking flag compatibility with
> previous versions of the linker, etc). Admittedly we'll be breaking
> output compatibility with this change regardless, so potentially
> having the flag as an escape hatch could be useful.
I hope we don't need to have a linker option. But if some not-so-old
versions of gdb / binutils programs / elfutils programs can't cope
with -2/-1/0 {{reserved_address}}, we may have to invent a linker option.
I hope GNU ld, gold and LLD can have a compatible option.
(As an LLD contributor, I'd be happy to implement the opinion in LLD)
