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LibELF: Don't copy read-only data sections

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The dynamic loader was mistakenly assuming that there are only two types
of program load headers: text (RX) and data (RW).

Now that we're linking with `-z separate-code`, we will also get some
read-onlydata (R) segments. These can be memory-mapped directly without
making a private per-process copy.

To solve this, the code now instead separates the headers into map/copy
instead of text/data. Writable segments get copied, while non-writable
segments get memory-mapped. :^)
This commit is contained in:
Andreas Kling 2021-09-01 01:15:32 +02:00
parent 10f5a046c4
commit 9206efaabe
1 changed files with 28 additions and 30 deletions
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58
Userland/Libraries/LibELF/DynamicLoader.cpp
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@ -246,8 +246,8 @@ void DynamicLoader::do_lazy_relocations()
void DynamicLoader::load_program_headers() void DynamicLoader::load_program_headers()
{ {
Vector<ProgramHeaderRegion> load_regions; Vector<ProgramHeaderRegion> load_regions;
Vector<ProgramHeaderRegion> text_regions; Vector<ProgramHeaderRegion> map_regions;
Vector<ProgramHeaderRegion> data_regions; Vector<ProgramHeaderRegion> copy_regions;
Optional<ProgramHeaderRegion> tls_region; Optional<ProgramHeaderRegion> tls_region;
Optional<ProgramHeaderRegion> relro_region; Optional<ProgramHeaderRegion> relro_region;
@ -263,10 +263,10 @@ void DynamicLoader::load_program_headers()
if (region.size_in_memory() == 0) if (region.size_in_memory() == 0)
return; return;
load_regions.append(region); load_regions.append(region);
if (region.is_executable()) { if (region.is_writable()) {
text_regions.append(region); copy_regions.append(region);
} else { } else {
data_regions.append(region); map_regions.append(region);
} }
} else if (region.is_dynamic()) { } else if (region.is_dynamic()) {
dynamic_region_desired_vaddr = region.desired_load_address(); dynamic_region_desired_vaddr = region.desired_load_address();
@ -276,15 +276,15 @@ void DynamicLoader::load_program_headers()
} }
}); });
VERIFY(!text_regions.is_empty() || !data_regions.is_empty()); VERIFY(!map_regions.is_empty() || !copy_regions.is_empty());
auto compare_load_address = [](ProgramHeaderRegion& a, ProgramHeaderRegion& b) { auto compare_load_address = [](ProgramHeaderRegion& a, ProgramHeaderRegion& b) {
return a.desired_load_address().as_ptr() < b.desired_load_address().as_ptr(); return a.desired_load_address().as_ptr() < b.desired_load_address().as_ptr();
}; };
quick_sort(load_regions, compare_load_address); quick_sort(load_regions, compare_load_address);
quick_sort(text_regions, compare_load_address); quick_sort(map_regions, compare_load_address);
quick_sort(data_regions, compare_load_address); quick_sort(copy_regions, compare_load_address);
// Process regions in order: .text, .data, .tls // Process regions in order: .text, .data, .tls
void* requested_load_address = m_elf_image.is_dynamic() ? nullptr : load_regions.first().desired_load_address().as_ptr(); void* requested_load_address = m_elf_image.is_dynamic() ? nullptr : load_regions.first().desired_load_address().as_ptr();
@ -295,9 +295,6 @@ void DynamicLoader::load_program_headers()
else else
reservation_mmap_flags |= MAP_FIXED; reservation_mmap_flags |= MAP_FIXED;
for (auto& text_region : text_regions)
VERIFY(!text_region.is_writable());
// First, we make a dummy reservation mapping, in order to allocate enough VM // First, we make a dummy reservation mapping, in order to allocate enough VM
// to hold all regions contiguously in the address space. // to hold all regions contiguously in the address space.
@ -320,27 +317,28 @@ void DynamicLoader::load_program_headers()
VERIFY_NOT_REACHED(); VERIFY_NOT_REACHED();
} }
for (auto& text_region : text_regions) { for (auto& region : map_regions) {
FlatPtr ph_text_desired_base = text_region.desired_load_address().get(); FlatPtr ph_desired_base = region.desired_load_address().get();
FlatPtr ph_text_base = text_region.desired_load_address().page_base().get(); FlatPtr ph_base = region.desired_load_address().page_base().get();
FlatPtr ph_text_end = ph_text_base + round_up_to_power_of_two(text_region.size_in_memory() + text_region.desired_load_address().get() - ph_text_base, PAGE_SIZE); FlatPtr ph_end = ph_base + round_up_to_power_of_two(region.size_in_memory() + region.desired_load_address().get() - ph_base, PAGE_SIZE);
// Now we can map the text segment at the reserved address. // Now we can map the text segment at the reserved address.
auto* text_segment_begin = (u8*)mmap_with_name( auto* segment_base = (u8*)mmap_with_name(
(u8*)reservation + ph_text_base - ph_load_base, (u8*)reservation + ph_base - ph_load_base,
ph_text_desired_base - ph_text_base + text_region.size_in_image(), ph_desired_base - ph_base + region.size_in_image(),
PROT_READ, PROT_READ,
MAP_FILE | MAP_SHARED | MAP_FIXED, MAP_FILE | MAP_SHARED | MAP_FIXED,
m_image_fd, m_image_fd,
VirtualAddress { text_region.offset() }.page_base().get(), VirtualAddress { region.offset() }.page_base().get(),
String::formatted("{}: .text", m_filename).characters()); String::formatted("{}: .text", m_filename).characters());
if (text_segment_begin == MAP_FAILED) { if (segment_base == MAP_FAILED) {
perror("mmap text"); perror("mmap non-writable");
VERIFY_NOT_REACHED(); VERIFY_NOT_REACHED();
} }
m_text_segments.append({ VirtualAddress { (FlatPtr)text_segment_begin }, ph_text_end - ph_text_base }); if (region.is_executable())
m_text_segments.append({ VirtualAddress { segment_base }, ph_end - ph_base });
} }
VERIFY(requested_load_address == nullptr || requested_load_address == reservation); VERIFY(requested_load_address == nullptr || requested_load_address == reservation);
@ -355,9 +353,9 @@ void DynamicLoader::load_program_headers()
else else
m_dynamic_section_address = dynamic_region_desired_vaddr; m_dynamic_section_address = dynamic_region_desired_vaddr;
for (auto& data_region : data_regions) { for (auto& region : copy_regions) {
FlatPtr ph_data_base = data_region.desired_load_address().page_base().get(); FlatPtr ph_data_base = region.desired_load_address().page_base().get();
FlatPtr ph_data_end = ph_data_base + round_up_to_power_of_two(data_region.size_in_memory() + data_region.desired_load_address().get() - ph_data_base, PAGE_SIZE); FlatPtr ph_data_end = ph_data_base + round_up_to_power_of_two(region.size_in_memory() + region.desired_load_address().get() - ph_data_base, PAGE_SIZE);
auto* data_segment_address = (u8*)reservation + ph_data_base - ph_load_base; auto* data_segment_address = (u8*)reservation + ph_data_base - ph_load_base;
size_t data_segment_size = ph_data_end - ph_data_base; size_t data_segment_size = ph_data_end - ph_data_base;
@ -373,19 +371,19 @@ void DynamicLoader::load_program_headers()
String::formatted("{}: .data", m_filename).characters()); String::formatted("{}: .data", m_filename).characters());
if (MAP_FAILED == data_segment) { if (MAP_FAILED == data_segment) {
perror("mmap data"); perror("mmap writable");
VERIFY_NOT_REACHED(); VERIFY_NOT_REACHED();
} }
VirtualAddress data_segment_start; VirtualAddress data_segment_start;
if (m_elf_image.is_dynamic()) if (m_elf_image.is_dynamic())
data_segment_start = VirtualAddress { (u8*)reservation + data_region.desired_load_address().get() }; data_segment_start = VirtualAddress { (u8*)reservation + region.desired_load_address().get() };
else else
data_segment_start = data_region.desired_load_address(); data_segment_start = region.desired_load_address();
VERIFY(data_segment_start.as_ptr() + data_region.size_in_memory() <= data_segment + data_segment_size); VERIFY(data_segment_start.as_ptr() + region.size_in_memory() <= data_segment + data_segment_size);
memcpy(data_segment_start.as_ptr(), (u8*)m_file_data + data_region.offset(), data_region.size_in_image()); memcpy(data_segment_start.as_ptr(), (u8*)m_file_data + region.offset(), region.size_in_image());
} }
// FIXME: Initialize the values in the TLS section. Currently, it is zeroed. // FIXME: Initialize the values in the TLS section. Currently, it is zeroed.