From 79c093226e609b99fa889f6e37480b92b399610d Mon Sep 17 00:00:00 2001
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From: Richard Purdie <richard.purdie@linuxfoundation.org>
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Date: Tue, 7 Mar 2017 21:08:34 +0000
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Subject: [PATCH] Avoid inflating file sizes needlessly and allow binaries to
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be stripped
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The current approach to changing sections in ET_DYN executables is to move
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the INTERP section to the end of the file. +This means changing PT_PHDR to
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add an extra PT_LOAD section so that the new section is mmaped into memory
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by the elf loader in the kernel. In order to extend PHDR, this means moving
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it to the end of the file.
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Its documented in BUGS there is a kernel 'bug' which means that if you have holes
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in memory between the base load address and the PT_LOAD segment that contains PHDR,
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it will pass an incorrect PHDR address to ld.so and fail to load the binary, segfaulting.
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To avoid this, the code currently inserts space into the binary to ensure that when
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loaded into memory there are no holes between the PT_LOAD sections. This inflates the
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binaries by many MBs in some cases. Whilst we could make them sparse, there is a second
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issue which is that strip can fail to process these binaries:
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$ strip fixincl
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Not enough room for program headers, try linking with -N
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[.note.ABI-tag]: Bad value
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This turns out to be due to libbfd not liking the relocated PHDR section either
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(https://github.com/NixOS/patchelf/issues/10).
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Instead this patch implements a different approach, leaving PHDR where it is but extending
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it in place to allow addition of a new PT_LOAD section. This overwrites sections in the
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binary but those get moved to the end of the file in the new PT_LOAD section.
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This is based on patches linked from the above github issue, however whilst the idea
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was good, the implementation wasn't correct and they've been rewritten here.
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Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
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Fetch from: https://github.com/NixOS/patchelf/commit/c4deb5e9e1ce9c98a48e0d5bb37d87739b8cfee4
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Backported to v0.9
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Signed-off-by: Conrad Ratschan <conrad.ratschan@rockwellcollins.com>
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---
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src/patchelf.cc | 71 ++++++++++++++++++++++++++++---------------------
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1 file changed, 40 insertions(+), 31 deletions(-)
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diff --git a/src/patchelf.cc b/src/patchelf.cc
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index 1d58061..c2147af 100644
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--- a/src/patchelf.cc
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+++ b/src/patchelf.cc
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@@ -209,6 +209,8 @@ private:
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string & replaceSection(const SectionName & sectionName,
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unsigned int size);
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+ bool haveReplacedSection(const SectionName & sectionName);
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+
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void writeReplacedSections(Elf_Off & curOff,
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Elf_Addr startAddr, Elf_Off startOffset);
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@@ -632,6 +634,15 @@ void ElfFile<ElfFileParamNames>::writeReplacedSections(Elf_Off & curOff,
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replacedSections.clear();
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}
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+template<ElfFileParams>
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+bool ElfFile<ElfFileParamNames>::haveReplacedSection(const SectionName & sectionName)
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+{
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+ ReplacedSections::iterator i = replacedSections.find(sectionName);
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+
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+ if (i != replacedSections.end())
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+ return true;
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+ return false;
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+}
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template<ElfFileParams>
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void ElfFile<ElfFileParamNames>::rewriteSectionsLibrary()
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@@ -648,52 +659,53 @@ void ElfFile<ElfFileParamNames>::rewriteSectionsLibrary()
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debug("last page is 0x%llx\n", (unsigned long long) startPage);
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+ /* Because we're adding a new section header, we're necessarily increasing
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+ the size of the program header table. This can cause the first section
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+ to overlap the program header table in memory; we need to shift the first
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+ few segments to someplace else. */
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+ /* Some sections may already be replaced so account for that */
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+ unsigned int i = 1;
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+ Elf_Addr pht_size = sizeof(Elf_Ehdr) + (phdrs.size() + 1)*sizeof(Elf_Phdr);
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+ while( shdrs[i].sh_addr <= pht_size && i < rdi(hdr->e_shnum) ) {
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+ if (not haveReplacedSection(getSectionName(shdrs[i])))
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+ replaceSection(getSectionName(shdrs[i]), shdrs[i].sh_size);
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+ i++;
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+ }
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- /* Compute the total space needed for the replaced sections and
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- the program headers. */
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- off_t neededSpace = (phdrs.size() + 1) * sizeof(Elf_Phdr);
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+ /* Compute the total space needed for the replaced sections */
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+ off_t neededSpace = 0;
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for (ReplacedSections::iterator i = replacedSections.begin();
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i != replacedSections.end(); ++i)
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neededSpace += roundUp(i->second.size(), sectionAlignment);
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debug("needed space is %d\n", neededSpace);
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-
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size_t startOffset = roundUp(fileSize, getPageSize());
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growFile(startOffset + neededSpace);
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-
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/* Even though this file is of type ET_DYN, it could actually be
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an executable. For instance, Gold produces executables marked
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- ET_DYN. In that case we can still hit the kernel bug that
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- necessitated rewriteSectionsExecutable(). However, such
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- executables also tend to start at virtual address 0, so
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+ ET_DYN as does LD when linking with pie. If we move PT_PHDR, it
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+ has to stay in the first PT_LOAD segment or any subsequent ones
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+ if they're continuous in memory due to linux kernel constraints
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+ (see BUGS). Since the end of the file would be after bss, we can't
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+ move PHDR there, we therefore choose to leave PT_PHDR where it is but
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+ move enough following sections such that we can add the extra PT_LOAD
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+ section to it. This PT_LOAD segment ensures the sections at the end of
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+ the file are mapped into memory for ld.so to process.
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+ We can't use the approach in rewriteSectionsExecutable()
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+ since DYN executables tend to start at virtual address 0, so
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rewriteSectionsExecutable() won't work because it doesn't have
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- any virtual address space to grow downwards into. As a
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- workaround, make sure that the virtual address of our new
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- PT_LOAD segment relative to the first PT_LOAD segment is equal
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- to its offset; otherwise we hit the kernel bug. This may
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- require creating a hole in the executable. The bigger the size
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- of the uninitialised data segment, the bigger the hole. */
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+ any virtual address space to grow downwards into. */
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if (isExecutable) {
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if (startOffset >= startPage) {
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debug("shifting new PT_LOAD segment by %d bytes to work around a Linux kernel bug\n", startOffset - startPage);
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- } else {
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- size_t hole = startPage - startOffset;
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- /* Print a warning, because the hole could be very big. */
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- fprintf(stderr, "warning: working around a Linux kernel bug by creating a hole of %zu bytes in ‘%s’\n", hole, fileName.c_str());
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- assert(hole % getPageSize() == 0);
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- /* !!! We could create an actual hole in the file here,
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- but it's probably not worth the effort. */
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- growFile(fileSize + hole);
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- startOffset += hole;
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}
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startPage = startOffset;
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}
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- /* Add a segment that maps the replaced sections and program
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- headers into memory. */
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+ /* Add a segment that maps the replaced sections into memory. */
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phdrs.resize(rdi(hdr->e_phnum) + 1);
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wri(hdr->e_phnum, rdi(hdr->e_phnum) + 1);
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Elf_Phdr & phdr = phdrs[rdi(hdr->e_phnum) - 1];
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@@ -706,15 +718,12 @@ void ElfFile<ElfFileParamNames>::rewriteSectionsLibrary()
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/* Write out the replaced sections. */
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- Elf_Off curOff = startOffset + phdrs.size() * sizeof(Elf_Phdr);
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+ Elf_Off curOff = startOffset;
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writeReplacedSections(curOff, startPage, startOffset);
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assert(curOff == startOffset + neededSpace);
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-
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- /* Move the program header to the start of the new area. */
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- wri(hdr->e_phoff, startOffset);
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-
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- rewriteHeaders(startPage);
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+ /* Write out the updated program and section headers */
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+ rewriteHeaders(hdr->e_phoff);
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}
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--
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2.17.1
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