[llvm-dev] [PATCH] D15965: Add support for dumping relocations in non-relocatable files

Thu Mar 24 13:35:06 PDT 2016

It sounds like what you’re asking is, rather that universally calling RelocationRef::getOffset inside llvm-objdump.cpp

I should:

*        Check if Obj “isRelocatableObject”

*        If it is, call RelocationRef::getOffset()

*        If it isn’t

o   Call RelocationRef::getAddress()

o   Build an ordered map of all sections and their bounds, check if the relocation lands within a section

o   Subtract the section address from the relocation address

Since getAddress isn’t in RelocationRef at the moment, this is going to have to be getAddress for MachO and COFF.

COFFObjectFile::getRelocationOffset returns R->VirtualAddress, so inside here I’m going to have to find the section it belongs to, add it to the result so it can later be subtracted off by llvm-objdump

MachOObjectFile::getRelocationOffset also returns getAnyRelocationAddress so I’ll have to also find its containing section and add this in just so llvm-objdump can later subtract it.

Can you detail why this is a better plan?

From: Rafael Espíndola [mailto:rafael.espindola at gmail.com] 
Sent: Thursday, March 24, 2016 3:13 PM
To: Colin LeMahieu <colinl at codeaurora.org>
Cc: llvm-commits <llvm-commits at lists.llvm.org>; Shankar Easwaran <shankare at codeaurora.org>; Hemant Kulkarni <khemant at codeaurora.org>; reviews+D15965+public+319b19c852e3c4e3 at reviews.llvm.org
Subject: RE: [PATCH] D15965: Add support for dumping relocations in non-relocatable files

On Mar 24, 2016 4:08 PM, "Colin LeMahieu" <colinl at codeaurora.org <mailto:colinl at codeaurora.org> > wrote:
>
> llvm-objdump wants to print the relocationOffset, so it calls RelocationRef::relocationOffset which calls ELFObjectFile::getRelocationOffset
>
> Since objdump wants the relocation offset, and this pipes down to ELFObjectFile::getRelocationOffset, at what point is the address supposed to be asked for?
>

In llvm-objdump. It can then search the sections to see if one happens to include that address.

> -----Original Message-----
> From: Rafael Espíndola [mailto:rafael.espindola at gmail.com <mailto:rafael.espindola at gmail.com> ]
> Sent: Thursday, March 24, 2016 2:45 PM
> To: reviews+D15965+public+319b19c852e3c4e3 at reviews.llvm.org <mailto:reviews%2BD15965%2Bpublic%2B319b19c852e3c4e3 at reviews.llvm.org> 
> Cc: colinl at codeaurora.org <mailto:colinl at codeaurora.org> ; Hemant Kulkarni <khemant at codeaurora.org <mailto:khemant at codeaurora.org> >; Shankar Easwaran <shankare at codeaurora.org <mailto:shankare at codeaurora.org> >; llvm-commits <llvm-commits at lists.llvm.org <mailto:llvm-commits at lists.llvm.org> >
> Subject: Re: [PATCH] D15965: Add support for dumping relocations in non-relocatable files
>
> On 24 March 2016 at 15:35, Shankar Easwaran <shankare at codeaurora.org <mailto:shankare at codeaurora.org> > wrote:
> > shankare added a reviewer: shankare.
> > shankare added a comment.
> >
> > http://www.sco.com/developers/gabi/latest/ch4.reloc.html
> >
> >   r_offset
> >   This member gives the location at which to apply the relocation action. For a relocatable file, the value is the byte offset from the beginning of the section to the storage unit affected by the relocation. For an executable file or a shared object, the value is the virtual address of the storage unit affected by the relocation.
> >
> > This shows that the assertion is incorrect, as the ELF ABI suggests reading relocation records is valid in Executables as well.
> >
> > While relocation offset is not looked up for shared libraries from
> > .rela.dyn and .rela.plt sections, there is a usecase where relocation
> > sections are generated with the linker switch **--emit-relocs**
>
> And those records have addresses, not offsets. Therefore the assertion is correct and you may *NOT* remove it.
>
> CHeers,
> Rafael
>

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