[RFC 4/4] ld: bfd: sframe: fix incorrect r_addend in RELA entries
Jens Remus
jremus@linux.ibm.com
Tue Mar 11 13:39:29 GMT 2025
On 11.03.2025 10:26, Jose E. Marchesi wrote:
>> On 10.03.2025 14:16, Jens Remus wrote:
>>> On 08.03.2025 08:38, Indu Bhagat wrote:
>>>> With the fix in GAS, we now use a different PC-relative RELA for
>>>> updating the SFrame FDE function start address: The value is the offset
>>>> of the start PC of the function from the start of the SFrame section.
>>>>
>>>> When RELAs are output (e.g. for relocatable links), there is need to
>>>> adjust the r_addend. This is because the r_addend values still have the
>>>> correct values for the _input_ SFrame section being linked / relocated.
>>>> The values must now be (before outputing the RELAs) with respect to the
>>>> _output_ SFrame section.
>>>>
>>>> PS: This patch should be merged with the previous commits before final
>>>> commit (Otherwise the tests will fail). It is currently a separate
>>>> patch as I would like to check if this is OK to do. If such a "addend
>>>> fixup" is risky or wrong, it seems we will need a new type of RELOC for
>>>> SFrame sections.
>>>
>>>> diff --git a/bfd/elf-sframe.c b/bfd/elf-sframe.c
>>>
>>>> @@ -574,6 +574,37 @@ _bfd_elf_sframe_section_offset (bfd *output_bfd ATTRIBUTE_UNUSED,
>>>> + out_fde_idx * sizeof (sframe_func_desc_entry));
>>>> }
>>>>
>>>> +/* Get the "canonicalized" addend for the symbol reference corresponding to the
>>>> + relocation at RELOC_INDEX. E.g., for the following reloc for the SFrame
>>>
>>> Nit: s/reloc/relocs/
>>>
>>>> + FDE function start address:
>>>> + Offset Type Sym. Name + Addend
>>>> + 00000000001c R_X86_64_PC32 .text + 1c
>>>> + 000000000030 R_X86_64_PC32 .text + 3b
>>>> + The canonicalized addend are 0 and b respectively as the relocs are for
>>>> + symbols (.text + 0) and (.text + b) respectively.
>>>
>>> Maybe it would help to use the function symbols foo and bar in the example:
>>>
>>> relocation at RELOC_INDEX. E.g., for the following annotated relocs for the SFrame
>>> FDE function start address of FDE[0] and FDE[1] for functions foo and bar:
>>> Offset Type Sym. Name + Addend
>>> 00000000001c R_X86_64_PC32 .text + 1c // FDE[0] for foo at .text + 0
>>> 000000000030 R_X86_64_PC32 .text + 3b // FDE[1] for bar at .text + b
>>> With:
>>> 1c = 0 + sizeof(sframe_header) + 0 * sizeof(sframe_fde)
>>> 3b = b + sizeof(sframe_header) + 1 * sizeof(sframe_fde)
>>> The canonicalized addend are 0 and b respectively as the relocs are for
>>> symbols (.text + 0) and (.text + b) respectively.
>>
>> Hmm, wait - there's a mix between .text and .sframe then, isn't there? That
>> would explain why custom handling is then necessary. Yet as said in the other
>> mail, custom handling should not be necessary. Not the least because other
>> ELF-consuming tools also need to work, not just what GNU binutils provides.
>
> [At a risk of being reiterative, but this also helps to my own
> understanding, because this is all very confusing :)]
>
> I think there is nothing special about that "mix". As far as I can tell
> this is just one rather peculiar particular case of a PCrel reloc.
> Suppose you have the typical:
>
> .text
> ...
> ldinsn OFFSET
>
> .data
> ...
> somedata:
>
> And then you install a relocation for the instruction's offset, which is
> PC-relative. You use an expression like:
>
> ldinsn somedata
>
> Then I would expect GAS to emit a PCrel with:
>
> .text.rela:
> P: offset_of_ldins_within_text
> S: .data
> A: offset_of_somedata_within_data
>
> With resolving expression:
>
> S + A - P
>
> to which the linker also adds .text, resulting in the expected:
>
> .data + offset_of_somedata_within_data - .text + offset_of_ldins_within_text
>
> i.e. the distance between somedata and the instruction which field is to
> be patched.
Isn't above in real world use cases actually already like what you
describe below for SFrame, in regard that the addend often consists of
the sum of the target offset and some source offset?
On s390x PC-relative addressing is relative to the instruction, so that:
.text.rela:
P: offset_of_ldins_within_text
S: .data
A: offset_of_somedata_within_data + offset_of_field_within_ldins
$ echo "brasl %r14,func@PLT" | as && objdump -dr a.out
...
0: c0 e5 00 00 00 00 brasl %r14,0x0
2: R_390_PLT32DBL func+0x2
~~~~
The +0x2 is the offset_of_field_within_ldins, which is very similar
the offset_of_fde_within_sframe in your below SFrame explanation.
On x86-64 PC-relative addressing is relative to the next instruction,
which makes it a bit less apparent:
$ echo "call func@PLT" | as && objdump -dr a.out
...
0: e8 00 00 00 00 call 0x5
1: R_X86_64_PLT32 func-0x4
~~~~
The -0x4 is the offset_of_field_from_insn_following_ldins.
> Now, for SFrame, the expression used wants to denote the distance
> between the beginning of some given function and the beginning of the
> .sframe section containing the FDE to patch. GAS is given the
> expression:
>
> .text + text_offset - .sframe
>
> And the assembler smartly uses a PCrel relocation (it has no other
> choice I suppose) such as:
>
> .sframe.rela:
> P: offset_of_fde_within_sframe
> S: .text
> A: offset_of_function_within_text + offset_of_fde_within_sframe
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> [I have been trying to find the code in GAS that actually does this,
> because it seems to me it must be some ad-hoc code, but couln't find
> it.]
>
> Which is a little weird, because it has the additional
> offset_of_fde_within_sframe, intended to basically annull the same value
> in P in the resolving function:
>
> S + A - P
>
> which is, with the linker-added .sframe:
>
> .text + (offset_of_funtion_within_text + offset_of_fde_within_sframe)
> - .sframe + offset_of_fde_within_sframe
>
> which becomes:
>
> .text + offset_of_funtion_within_text - .sframe
>
> which is the desired distance between the beginning of the function and
> the beginning of the .sframe section.
>
> I don't think ELF consuming tools that just apply relocations need any
> special code to solve these relocs.
I agree. See the s390x and x86-64 examples above, which do not require
any special processing.
> But due how the SFrame format works, as Jens explained, tools that merge
> SFrame input sections into a relocatable output section need to
> basically recreate the relocations for the FDEs in the output. If this
> merging consisted (like with ehframe) just in collating the input
> sections together, then no special casing would be needed. But merging
> SFrame sections imply that the offset of the FDEs wrt the containing
> output .sframe will likely be different. This offset is embedded in the
> input relocation's offset and addend, as shown above. So the merging
> tool has no other choice that to "remove" the old FDE offset from the
> input relocation (from both addend and offset) and "insert" the new FDE
> offset (in the resulting merged .sframe section in the output) in both
> addend and offset. Tools that merge SFrame sections must use
> sframe-specific logic anyway, so I don't think this is a big deal...
>
> The other changes in generic code in Indu's patch, as far as I can see,
> are necessary because the linker assumes that the input section contents
> are merged by stocking them (or a subset of them) in the merged output
> section. This is not so in SFrame, so thats why Indu's patch makes ld
> to not increase that offset within the output section every iteration in
> the loop, if the sections being merged are .sframe. It is simply that
> until now no format required this sort of merging, where I11,I12,I13 and
> I21,I22,I23 areas in two input sections will be merged into for example
> I11,I21,I12,I22,I13,I23 in the output section.
Very good explanation! This is the main difference, why SFrame requires
special processing - but only for merging of .sframe sections.
Regards,
Jens
--
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com
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