| From 8d2cb4f9ab8d564904c292099a022ffb3cccd52d Mon Sep 17 00:00:00 2001 |
| From: Jason <otherjason@nodomain.com> |
| Date: Fri, 2 Dec 2022 10:01:41 -0500 |
| Subject: [PATCH] Fix bug in file shifting that could cause conflicting PT_LOAD |
| segments |
| |
| When a section in the file needs to be enlarged (e.g. to accommodate |
| setting a larger RPATH), shiftFile() is used to shift all content |
| following the growing section to a later position in the file. |
| |
| Commit 109b771f53ee3d37ede8c0f165665605183c0975 introduced logic to |
| ensure that, after the segment split, no sections span multiple |
| segments. This is done by sliding the portion of the segment after the |
| split point later in the file, then adding a new PT_LOAD segment that |
| contains the preceding data plus the extra room that is being added. The |
| existing implementation does this by simply adding |
| `extraPages*getPageSize()` bytes to the number of bytes ahead of the |
| split point in the segment. |
| |
| However, this approach can result in two PT_LOAD segments that overlap |
| when page boundaries are taken into account. As an example, this PT_LOAD |
| section (taken from a Python 3.10 binary): |
| |
| LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000 |
| 0x0000000000000948 0x0000000000000948 R E 0x200000 |
| |
| is split into the following two sections: |
| |
| LOAD 0x0000000000000000 0x00000000003ff000 0x00000000003ff000 |
| 0x0000000000001594 0x0000000000001594 R E 0x1000 |
| LOAD 0x0000000000001594 0x0000000000400594 0x0000000000400594 |
| 0x00000000000003b4 0x00000000000003b4 R E 0x1000 |
| |
| Note that the two PT_LOAD sections both contain the memory page at |
| address 0x400000. The Linux kernel's ELF loader (at least as of v4.18) |
| does not accept this as a valid ELF executable, triggering a segfault |
| with si_code=SI_KERNEL immediately when the binary is executed. |
| |
| The fix here is to set the length of the segment that comes before the |
| split point more carefully; instead of adding `extraPages*getPageSize()` |
| bytes to the portion of the segment that came before the split, the |
| actual number of padding bytes that were needed (before rounding up to |
| the next multiple of the page size) are used. This avoids the overlap |
| in the PT_LOAD segments and makes the output files executable again. |
| --- |
| src/patchelf.cc | 10 ++++++---- |
| src/patchelf.h | 2 +- |
| 2 files changed, 7 insertions(+), 5 deletions(-) |
| |
| Upstream-Status: Submitted [https://github.com/NixOS/patchelf/pull/447] |
| Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org> |
| |
| Index: git/src/patchelf.cc |
| =================================================================== |
| --- git.orig/src/patchelf.cc |
| +++ git/src/patchelf.cc |
| @@ -432,7 +432,7 @@ static uint64_t roundUp(uint64_t n, uint |
| |
| |
| template<ElfFileParams> |
| -void ElfFile<ElfFileParamNames>::shiftFile(unsigned int extraPages, size_t startOffset) |
| +void ElfFile<ElfFileParamNames>::shiftFile(unsigned int extraPages, size_t startOffset, size_t extraBytes) |
| { |
| assert(startOffset >= sizeof(Elf_Ehdr)); |
| |
| @@ -508,7 +508,7 @@ void ElfFile<ElfFileParamNames>::shiftFi |
| wri(phdr.p_offset, phdrs.at(splitIndex).p_offset - splitShift - shift); |
| wri(phdr.p_paddr, phdrs.at(splitIndex).p_paddr - splitShift - shift); |
| wri(phdr.p_vaddr, phdrs.at(splitIndex).p_vaddr - splitShift - shift); |
| - wri(phdr.p_filesz, wri(phdr.p_memsz, splitShift + shift)); |
| + wri(phdr.p_filesz, wri(phdr.p_memsz, splitShift + extraBytes)); |
| wri(phdr.p_flags, PF_R | PF_W); |
| wri(phdr.p_align, getPageSize()); |
| } |
| @@ -898,12 +898,14 @@ void ElfFile<ElfFileParamNames>::rewrite |
| neededSpace += sizeof(Elf_Phdr); |
| debug("needed space is %d\n", neededSpace); |
| |
| - unsigned int neededPages = roundUp(neededSpace - startOffset, getPageSize()) / getPageSize(); |
| + /* Calculate how many bytes are needed out of the additional pages. */ |
| + size_t extraSpace = neededSpace - startOffset; |
| + unsigned int neededPages = roundUp(extraSpace, getPageSize()) / getPageSize(); |
| debug("needed pages is %d\n", neededPages); |
| if (neededPages * getPageSize() > firstPage) |
| error("virtual address space underrun!"); |
| |
| - shiftFile(neededPages, startOffset); |
| + shiftFile(neededPages, startOffset, extraSpace); |
| |
| firstPage -= neededPages * getPageSize(); |
| startOffset += neededPages * getPageSize(); |
| Index: git/src/patchelf.h |
| =================================================================== |
| --- git.orig/src/patchelf.h |
| +++ git/src/patchelf.h |
| @@ -77,7 +77,7 @@ private: |
| |
| void sortShdrs(); |
| |
| - void shiftFile(unsigned int extraPages, size_t sizeOffset); |
| + void shiftFile(unsigned int extraPages, size_t sizeOffset, size_t extraBytes); |
| |
| std::string getSectionName(const Elf_Shdr & shdr) const; |
| |