[lld] r244691 - COFF: Align sections to 512-byte boundaries on disk.
Rui Ueyama via llvm-commits
llvm-commits at lists.llvm.org
Wed Aug 12 22:59:17 PDT 2015
What I don't understand is that why the offset from the beginning of the
file must be multiple of page size in order to avoid full copy. Windows
requires all sections to be aligned at least 4K in memory and 512 bytes on
file, and I don't see any problem there.
Let's say we have two sections, A and B, whose sizes are 1024B and 4096B,
respectively. We also assume that A's offset from the beginning of file is
4096, and B's 5120. The loader can map offset 4096 to 8192 of the file to
some page, and 5120 to 9216 to other page. Why can't that?
On Thu, Aug 13, 2015 at 2:44 PM, Sean Silva <chisophugis at gmail.com> wrote:
>
>
> On Wed, Aug 12, 2015 at 7:17 PM, Rui Ueyama <ruiu at google.com> wrote:
>
>> I didn't test to see if this change ever has a negative impact on memory
>> usage, but my guess is that's very unlikely because this layout is the same
>> as what MSVC linker creates. If this is inefficient, virtually all Windows
>> executables are being suffered by that, which is unlikely. My understanding
>> is that the kernel maps each section separately to a memory address, so
>> file offset of each section can be given independently from other sections.
>>
>
> The mapping is done at the granularity of aligned 4K pages minimum (this
> is just how the x86 hardware page table mechanism works). A piece of the
> file cannot be moved by an amount that is not a multiple of 4K without a
> full copy.
>
> The only way this could (in the usual case) not have a large overhead is
> for the kernel to do a crazy hack like have special paging semantics for
> files that are executables. This means that when LLD finishes working on a
> memory mapped file, if a section is not 4K aligned at least, then the
> kernel has to then do a copy to make the file conform the the actual memory
> layout it needs to have in the paging subsystem.
>
> Or does windows make full copies of sections always? In other words
> processes don't share e.g. readonly text?
>
> In ELF, the offset in the file and the offset in memory are required to be
> congruent modulo the alignment (see the documentation of p_align in
> http://www.sco.com/developers/gabi/latest/ch5.pheader.html), precisely to
> avoid the need to do crazy hacks like this when loading the program.
>
> You can see that Linux will reject the binary:
> http://lxr.free-electrons.com/source/fs/binfmt_elf.c#L664
> (load_elf_binary)
> -> http://lxr.free-electrons.com/source/fs/binfmt_elf.c#L336 (elf_map)
> -> http://lxr.free-electrons.com/source/mm/util.c#L306 (vm_mmap)
> Notice:
> 312 if (unlikely(offset & ~PAGE_MASK))
> 313 return -EINVAL;
>
> FreeBSD is more lenient, but you can see that the kernel does not like the
> situation when this is violated:
> http://src.illumos.org/source/xref/freebsd-head/sys/kern/imgact_elf.c#593
> (__elfN(load_file))
> -->
> http://src.illumos.org/source/xref/freebsd-head/sys/kern/imgact_elf.c#467
> (__elfN(load_section))
> -->
> http://src.illumos.org/source/xref/freebsd-head/sys/kern/imgact_elf.c#398
> (__elfN(map_insert))
> 423 /*
> 424 * The mapping is not page aligned. This means we have
> 425 * to copy the data. Sigh.
> 426 */
>
> -- Sean Silva
>
>
>>
>> On Wed, Aug 12, 2015 at 5:31 PM, Sean Silva <chisophugis at gmail.com>
>> wrote:
>>
>>>
>>>
>>> On Tue, Aug 11, 2015 at 4:09 PM, Rui Ueyama via llvm-commits <
>>> llvm-commits at lists.llvm.org> wrote:
>>>
>>>> Author: ruiu
>>>> Date: Tue Aug 11 18:09:00 2015
>>>> New Revision: 244691
>>>>
>>>> URL: http://llvm.org/viewvc/llvm-project?rev=244691&view=rev
>>>> Log:
>>>> COFF: Align sections to 512-byte boundaries on disk.
>>>>
>>>> Sections must start at page boundaries in memory, but they
>>>> can be aligned to sector boundaries (512-bytes) on disk.
>>>> We aligned them to 4096-byte boundaries even on disk, so we
>>>> wasted disk space a bit.
>>>>
>>>
>>> This will likely force the kernel to copy or otherwise do unnecessary
>>> work when loading. Are you sure that isn't happening? The kernel ideally
>>> wants to just create a couple page table entries. But if it needs to move
>>> things around at <4K granularity to make them properly aligned to their
>>> load address when loading (like this patch I think causes) then it will
>>> need to do copies.
>>>
>>> This can likely be checked by looking for an increase in real memory
>>> usage for the system when the new binaries are loaded (vs. the old
>>> page-aligned ones), since the kernel will have a copy sitting in page cache
>>> and a copy for alignment mapped into the process address space;
>>> alternatively, you can check for the slowdown from the kernel copies when
>>> faulting the memory into the process's address space (or (less likely) it
>>> may do the copies eagerly which should be easy to measure too).
>>>
>>> -- Sean Silva
>>>
>>>
>>>>
>>>> Modified:
>>>> lld/trunk/COFF/Writer.cpp
>>>> lld/trunk/test/COFF/baserel.test
>>>> lld/trunk/test/COFF/hello32.test
>>>>
>>>> Modified: lld/trunk/COFF/Writer.cpp
>>>> URL:
>>>> http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/Writer.cpp?rev=244691&r1=244690&r2=244691&view=diff
>>>>
>>>> ==============================================================================
>>>> --- lld/trunk/COFF/Writer.cpp (original)
>>>> +++ lld/trunk/COFF/Writer.cpp Tue Aug 11 18:09:00 2015
>>>> @@ -37,8 +37,7 @@ using namespace lld;
>>>> using namespace lld::coff;
>>>>
>>>> static const int PageSize = 4096;
>>>> -static const int FileAlignment = 512;
>>>> -static const int SectionAlignment = 4096;
>>>> +static const int SectorSize = 512;
>>>> static const int DOSStubSize = 64;
>>>> static const int NumberfOfDataDirectory = 16;
>>>>
>>>> @@ -174,7 +173,7 @@ void OutputSection::addChunk(Chunk *C) {
>>>> Off += C->getSize();
>>>> Header.VirtualSize = Off;
>>>> if (C->hasData())
>>>> - Header.SizeOfRawData = RoundUpToAlignment(Off, FileAlignment);
>>>> + Header.SizeOfRawData = RoundUpToAlignment(Off, SectorSize);
>>>> }
>>>>
>>>> void OutputSection::addPermissions(uint32_t C) {
>>>> @@ -507,15 +506,14 @@ void Writer::createSymbolAndStringTable(
>>>> // We position the symbol table to be adjacent to the end of the
>>>> last section.
>>>> uint64_t FileOff =
>>>> LastSection->getFileOff() +
>>>> - RoundUpToAlignment(LastSection->getRawSize(), FileAlignment);
>>>> + RoundUpToAlignment(LastSection->getRawSize(), SectorSize);
>>>> if (!OutputSymtab.empty()) {
>>>> PointerToSymbolTable = FileOff;
>>>> FileOff += OutputSymtab.size() * sizeof(coff_symbol16);
>>>> }
>>>> if (!Strtab.empty())
>>>> FileOff += Strtab.size() + 4;
>>>> - FileSize = SizeOfHeaders +
>>>> - RoundUpToAlignment(FileOff - SizeOfHeaders,
>>>> FileAlignment);
>>>> + FileSize = RoundUpToAlignment(FileOff, SectorSize);
>>>> }
>>>>
>>>> // Visits all sections to assign incremental, non-overlapping RVAs and
>>>> @@ -526,9 +524,9 @@ void Writer::assignAddresses() {
>>>> sizeof(coff_section) * OutputSections.size();
>>>> SizeOfHeaders +=
>>>> Config->is64() ? sizeof(pe32plus_header) : sizeof(pe32_header);
>>>> - SizeOfHeaders = RoundUpToAlignment(SizeOfHeaders, PageSize);
>>>> + SizeOfHeaders = RoundUpToAlignment(SizeOfHeaders, SectorSize);
>>>> uint64_t RVA = 0x1000; // The first page is kept unmapped.
>>>> - uint64_t FileOff = SizeOfHeaders;
>>>> + FileSize = SizeOfHeaders;
>>>> // Move DISCARDABLE (or non-memory-mapped) sections to the end of
>>>> file because
>>>> // the loader cannot handle holes.
>>>> std::stable_partition(
>>>> @@ -539,13 +537,11 @@ void Writer::assignAddresses() {
>>>> if (Sec->getName() == ".reloc")
>>>> addBaserels(Sec);
>>>> Sec->setRVA(RVA);
>>>> - Sec->setFileOffset(FileOff);
>>>> + Sec->setFileOffset(FileSize);
>>>> RVA += RoundUpToAlignment(Sec->getVirtualSize(), PageSize);
>>>> - FileOff += RoundUpToAlignment(Sec->getRawSize(), FileAlignment);
>>>> + FileSize += RoundUpToAlignment(Sec->getRawSize(), SectorSize);
>>>> }
>>>> SizeOfImage = SizeOfHeaders + RoundUpToAlignment(RVA - 0x1000,
>>>> PageSize);
>>>> - FileSize = SizeOfHeaders +
>>>> - RoundUpToAlignment(FileOff - SizeOfHeaders,
>>>> FileAlignment);
>>>> }
>>>>
>>>> template <typename PEHeaderTy> void Writer::writeHeader() {
>>>> @@ -584,8 +580,8 @@ template <typename PEHeaderTy> void Writ
>>>> Buf += sizeof(*PE);
>>>> PE->Magic = Config->is64() ? PE32Header::PE32_PLUS :
>>>> PE32Header::PE32;
>>>> PE->ImageBase = Config->ImageBase;
>>>> - PE->SectionAlignment = SectionAlignment;
>>>> - PE->FileAlignment = FileAlignment;
>>>> + PE->SectionAlignment = PageSize;
>>>> + PE->FileAlignment = SectorSize;
>>>> PE->MajorImageVersion = Config->MajorImageVersion;
>>>> PE->MinorImageVersion = Config->MinorImageVersion;
>>>> PE->MajorOperatingSystemVersion = Config->MajorOSVersion;
>>>>
>>>> Modified: lld/trunk/test/COFF/baserel.test
>>>> URL:
>>>> http://llvm.org/viewvc/llvm-project/lld/trunk/test/COFF/baserel.test?rev=244691&r1=244690&r2=244691&view=diff
>>>>
>>>> ==============================================================================
>>>> --- lld/trunk/test/COFF/baserel.test (original)
>>>> +++ lld/trunk/test/COFF/baserel.test Tue Aug 11 18:09:00 2015
>>>> @@ -61,7 +61,7 @@
>>>> # BASEREL-HEADER-NEXT: VirtualSize: 0x20
>>>> # BASEREL-HEADER-NEXT: VirtualAddress: 0x5000
>>>> # BASEREL-HEADER-NEXT: RawDataSize: 512
>>>> -# BASEREL-HEADER-NEXT: PointerToRawData: 0x1800
>>>> +# BASEREL-HEADER-NEXT: PointerToRawData: 0xC00
>>>> # BASEREL-HEADER-NEXT: PointerToRelocations: 0x0
>>>> # BASEREL-HEADER-NEXT: PointerToLineNumbers: 0x0
>>>> # BASEREL-HEADER-NEXT: RelocationCount: 0
>>>>
>>>> Modified: lld/trunk/test/COFF/hello32.test
>>>> URL:
>>>> http://llvm.org/viewvc/llvm-project/lld/trunk/test/COFF/hello32.test?rev=244691&r1=244690&r2=244691&view=diff
>>>>
>>>> ==============================================================================
>>>> --- lld/trunk/test/COFF/hello32.test (original)
>>>> +++ lld/trunk/test/COFF/hello32.test Tue Aug 11 18:09:00 2015
>>>> @@ -38,8 +38,8 @@ HEADER-NEXT: MajorImageVersion: 0
>>>> HEADER-NEXT: MinorImageVersion: 0
>>>> HEADER-NEXT: MajorSubsystemVersion: 6
>>>> HEADER-NEXT: MinorSubsystemVersion: 0
>>>> -HEADER-NEXT: SizeOfImage: 20480
>>>> -HEADER-NEXT: SizeOfHeaders: 4096
>>>> +HEADER-NEXT: SizeOfImage: 16896
>>>> +HEADER-NEXT: SizeOfHeaders: 512
>>>> HEADER-NEXT: Subsystem: IMAGE_SUBSYSTEM_WINDOWS_CUI (0x3)
>>>> HEADER-NEXT: Characteristics [ (0x8140)
>>>> HEADER-NEXT: IMAGE_DLL_CHARACTERISTICS_DYNAMIC_BASE (0x40)
>>>>
>>>>
>>>> _______________________________________________
>>>> llvm-commits mailing list
>>>> llvm-commits at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>>>
>>>
>>>
>>
>
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