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/*
* Copyright (C) 2012 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include "elf_stripper.h"
#include <unistd.h>
#include <sys/types.h>
#include <memory>
#include <vector>
#include "base/logging.h"
#include "base/stringprintf.h"
#include "elf_file.h"
#include "elf_utils.h"
#include "utils.h"
namespace art {
bool ElfStripper::Strip(File* file, std::string* error_msg) {
std::unique_ptr<ElfFile> elf_file(ElfFile::Open(file, true, false, error_msg));
if (elf_file.get() == nullptr) {
return false;
}
// ELF files produced by MCLinker look roughly like this
//
// +------------+
// | Elf32_Ehdr | contains number of Elf32_Shdr and offset to first
// +------------+
// | Elf32_Phdr | program headers
// | Elf32_Phdr |
// | ... |
// | Elf32_Phdr |
// +------------+
// | section | mixture of needed and unneeded sections
// +------------+
// | section |
// +------------+
// | ... |
// +------------+
// | section |
// +------------+
// | Elf32_Shdr | section headers
// | Elf32_Shdr |
// | ... | contains offset to section start
// | Elf32_Shdr |
// +------------+
//
// To strip:
// - leave the Elf32_Ehdr and Elf32_Phdr values in place.
// - walk the sections making a new set of Elf32_Shdr section headers for what we want to keep
// - move the sections are keeping up to fill in gaps of sections we want to strip
// - write new Elf32_Shdr section headers to end of file, updating Elf32_Ehdr
// - truncate rest of file
//
std::vector<Elf32_Shdr> section_headers;
std::vector<Elf32_Word> section_headers_original_indexes;
section_headers.reserve(elf_file->GetSectionHeaderNum());
Elf32_Shdr& string_section = elf_file->GetSectionNameStringSection();
for (Elf32_Word i = 0; i < elf_file->GetSectionHeaderNum(); i++) {
Elf32_Shdr& sh = elf_file->GetSectionHeader(i);
const char* name = elf_file->GetString(string_section, sh.sh_name);
if (name == NULL) {
CHECK_EQ(0U, i);
section_headers.push_back(sh);
section_headers_original_indexes.push_back(0);
continue;
}
if (StartsWith(name, ".debug")
|| (strcmp(name, ".strtab") == 0)
|| (strcmp(name, ".symtab") == 0)) {
continue;
}
section_headers.push_back(sh);
section_headers_original_indexes.push_back(i);
}
CHECK_NE(0U, section_headers.size());
CHECK_EQ(section_headers.size(), section_headers_original_indexes.size());
// section 0 is the NULL section, sections start at offset of first section
Elf32_Off offset = elf_file->GetSectionHeader(1).sh_offset;
for (size_t i = 1; i < section_headers.size(); i++) {
Elf32_Shdr& new_sh = section_headers[i];
Elf32_Shdr& old_sh = elf_file->GetSectionHeader(section_headers_original_indexes[i]);
CHECK_EQ(new_sh.sh_name, old_sh.sh_name);
if (old_sh.sh_addralign > 1) {
offset = RoundUp(offset, old_sh.sh_addralign);
}
if (old_sh.sh_offset == offset) {
// already in place
offset += old_sh.sh_size;
continue;
}
// shift section earlier
memmove(elf_file->Begin() + offset,
elf_file->Begin() + old_sh.sh_offset,
old_sh.sh_size);
new_sh.sh_offset = offset;
offset += old_sh.sh_size;
}
Elf32_Off shoff = offset;
size_t section_headers_size_in_bytes = section_headers.size() * sizeof(Elf32_Shdr);
memcpy(elf_file->Begin() + offset, §ion_headers[0], section_headers_size_in_bytes);
offset += section_headers_size_in_bytes;
elf_file->GetHeader().e_shnum = section_headers.size();
elf_file->GetHeader().e_shoff = shoff;
int result = ftruncate(file->Fd(), offset);
if (result != 0) {
*error_msg = StringPrintf("Failed to truncate while stripping ELF file: '%s': %s",
file->GetPath().c_str(), strerror(errno));
return false;
}
return true;
}
} // namespace art
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