[lld] r282705 - [ELF] Change the way we compute file offsets

[Rafael Espíndola via llvm-commits]

Thanks!

On 29 September 2016 at 02:20, Eugene Leviant via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: evgeny777
> Date: Thu Sep 29 04:20:33 2016
> New Revision: 282705
>
> URL: http://llvm.org/viewvc/llvm-project?rev=282705&view=rev
> Log:
> [ELF] Change the way we compute file offsets
>
> If we two sections reside in the same PT_LOAD segment,
> we compute second section using the following formula:
> Off2 = Off1 + VA2 - VA1. This allows OS kernel allocating
> sections correctly when loading an image.
>
> Differential revision: https://reviews.llvm.org/D25014
>
> Modified:
>     lld/trunk/ELF/OutputSections.h
>     lld/trunk/ELF/Writer.cpp
>     lld/trunk/test/ELF/linkerscript/outsections-addr.s
>
> Modified: lld/trunk/ELF/OutputSections.h
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/OutputSections.h?rev=282705&r1=282704&r2=282705&view=diff
> ==============================================================================
> --- lld/trunk/ELF/OutputSections.h (original)
> +++ lld/trunk/ELF/OutputSections.h Thu Sep 29 04:20:33 2016
> @@ -77,6 +77,7 @@ public:
>    void setVA(uintX_t VA) { Header.sh_addr = VA; }
>    uintX_t getVA() const { return Header.sh_addr; }
>    void setFileOffset(uintX_t Off) { Header.sh_offset = Off; }
> +  uintX_t getFileOffset() { return Header.sh_offset; }
>    void setSHName(unsigned Val) { Header.sh_name = Val; }
>    void writeHeaderTo(Elf_Shdr *SHdr);
>    StringRef getName() { return Name; }
> @@ -108,6 +109,14 @@ public:
>    // Typically the first section of each PT_LOAD segment has this flag.
>    bool PageAlign = false;
>
> +  // Pointer to the first section in PT_LOAD segment, which this section
> +  // also resides in. This field is used to correctly compute file offset
> +  // of a section. When two sections share the same load segment, difference
> +  // between their file offsets should be equal to difference between their
> +  // virtual addresses. To compute some section offset we use the following
> +  // formula: Off = Off_first + VA - VA_first.
> +  OutputSectionBase<ELFT> *FirstInPtLoad = nullptr;
> +
>    virtual void finalize() {}
>    virtual void finalizePieces() {}
>    virtual void assignOffsets() {}
>
> Modified: lld/trunk/ELF/Writer.cpp
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Writer.cpp?rev=282705&r1=282704&r2=282705&view=diff
> ==============================================================================
> --- lld/trunk/ELF/Writer.cpp (original)
> +++ lld/trunk/ELF/Writer.cpp Thu Sep 29 04:20:33 2016
> @@ -542,6 +542,8 @@ void PhdrEntry<ELFT>::add(OutputSectionB
>    if (!First)
>      First = Sec;
>    H.p_align = std::max<typename ELFT::uint>(H.p_align, Sec->getAlignment());
> +  if (H.p_type == PT_LOAD && !Config->OFormatBinary)
> +    Sec->FirstInPtLoad = First;
>  }
>
>  template <class ELFT>
> @@ -1190,7 +1192,13 @@ static uintX_t getFileAlignment(uintX_t
>    // and does not need any other offset adjusting.
>    if (Config->Relocatable || !(Sec->getFlags() & SHF_ALLOC))
>      return Off;
> -  return alignTo(Off, Config->MaxPageSize, Sec->getVA());
> +
> +  OutputSectionBase<ELFT> *First = Sec->FirstInPtLoad;
> +  // If two sections share the same PT_LOAD the file offset is calculated using
> +  // this formula: Off2 = Off1 + (VA2 - VA1).
> +  if (!First || Sec == First)
> +    return alignTo(Off, Target->MaxPageSize, Sec->getVA());
> +  return First->getFileOffset() + Sec->getVA() - First->getVA();
>  }
>
>  template <class ELFT, class uintX_t>
>
> Modified: lld/trunk/test/ELF/linkerscript/outsections-addr.s
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/linkerscript/outsections-addr.s?rev=282705&r1=282704&r2=282705&view=diff
> ==============================================================================
> --- lld/trunk/test/ELF/linkerscript/outsections-addr.s (original)
> +++ lld/trunk/test/ELF/linkerscript/outsections-addr.s Thu Sep 29 04:20:33 2016
> @@ -81,7 +81,7 @@
>  #CHECK:      SHF_ALLOC
>  #CHECK:    ]
>  #CHECK:    Address: 0x4008
> -#CHECK:    Offset: 0x2008
> +#CHECK:    Offset: 0x3008
>  #CHECK:    Size: 8
>  #CHECK:    Link: 0
>  #CHECK:    Info: 0
> @@ -96,7 +96,7 @@
>  #CHECK:      SHF_ALLOC
>  #CHECK:    ]
>  #CHECK:    Address: 0x5010
> -#CHECK:    Offset: 0x2010
> +#CHECK:    Offset: 0x4010
>  #CHECK:    Size: 8
>  #CHECK:    Link: 0
>  #CHECK:    Info: 0
>
>
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