Light ELF: exploring potential size reduction

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Fangrui Song

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Apr 1, 2024, 3:22:02 AMApr 1
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ELF's design emphasizes natural size and alignment guidelines for its control structures. While ensured efficient processing in the old days, this can lead to larger file sizes. I propose "Light ELF" (EV_LIGHT, version 2) – a whimsical exploration inspired by Light Elves of Tolkien's legendarium (who had seen the light of the Two Trees in Valinor).

In a light ELF file, the e_version member of the ELF header is set to 2. EV_CURRENT remains 1 for backward compatibility.

When linking a program, traditional ELF (version 1) and light ELF (version 2) files can be mixed together.

Relocations

Light ELF utilizes CREL for relocations. REL and RELA from traditional ELF are unused.

Existing lazy binding schemes rely on random access to relocation entries within the DT_JMPREL table. Due to CREL's sequential nature, keeping lazy binding requires a memory allocation that holds decoded JUMP_SLOT relocations.

Section header table

Traditional ELF (version 1) section header tables can be large. Light ELF addresses this through a compact section header table format signaled by e_shentsize == 0 in the ELF header.

A compact section header table for ELF contains the detail. Its current version is copied below for your convenience.

nshdr denotes the number of sections (including SHN_UNDEF). The section header table (located at e_shoff) begins with nshdr Elf_Word values. These values specify the offset of each section header relative to e_shoff.

Following these offsets, nshdr section headers are encoded. Each header begins with a presence byte indicating which subsequent Elf_Shdr members use explicit values vs. defaults:

  • sh_name, ULEB128 encoded
  • sh_type, ULEB128 encoded (if presence & 1), defaults to SHT_PROGBITS
  • sh_flags, ULEB128 encoded (if presence & 2), defaults to 0
  • sh_addr, ULEB128 encoded (if presence & 4), defaults to 0
  • sh_offset, ULEB128 encoded
  • sh_size, ULEB128 encoded (if presence & 8), defaults to 0
  • sh_link, ULEB128 encoded (if presence & 16), defaults to 0
  • sh_info, ULEB128 encoded (if presence & 32), defaults to 0
  • sh_addralign, ULEB128 encoded as log2 value (if presence & 64), defaults to 1
  • sh_entsize, ULEB128 encoded (if presence & 128), defaults to 0

In traditional ELF, sh_addralign can be 0 or a positive integral power of two, where 0 and 1 mean the section has no alignment constraints. While the compact encoding cannot encode sh_addralign value of 0, there is no loss of generality.

Example C++ code that decodes a specific section header:

const uint8_t *sht = base + ehdr->e_shoff;
const uint8_t *p = sht + ((Elf_Word*)sht)[i];
uint8_t presence = *p++;
Elf_Shdr shdr = {};
shdr.sh_name = readULEB128(p);
shdr.sh_type = presence & 1 ? readULEB128(p) : ELF::SHT_PROGBITS;
shdr.sh_flags = presence & 2 ? readULEB128(p) : 0;
shdr.sh_addr = presence & 4 ? readULEB128(p) : 0;
shdr.sh_offset = readULEB128(p);
shdr.sh_size = presence & 8 ? readULEB128(p) : 0;
shdr.sh_link = presence & 16 ? readULEB128(p) : 0;
shdr.sh_info = presence & 32 ? readULEB128(p) : 0;
shdr.sh_addralign = presence & 64 ? 1UL << readULEB128(p) : 1;
shdr.sh_entsize = presence & 128 ? readULEB128(p) : 0;

While the current format allows for O(1) in-place random access of section headers using offsets at the beginning of the table, this access pattern seems uncommon in practice. At least, I haven't encountered (or remembered) any instances within the llvm-project codebase. Therefore, I'm considering removing this functionality.

In a release build of llvm-project (-O3 -ffunction-sections -fdata-sections -Wa,--crel, the traditional section header tables occupy 16.4% of the .o file size while the compact section header table drastically reduces the ratio to 4.7%.

Symbol table

Like other sections, symbol table and string table sections (SHT_SYMTAB and SHT_STRTAB) can be compressed through SHF_COMPRESSED. However, compressing the dynamic symbol table (.dynsym) and its associated string table (.dynstr) is not recommended.

Symbol table sections have a non-zero sh_entsize, which remains unchanged after compression.

The string table, which stores symbol names (also section names in LLVM output), is typically much larger than the symbol table itself. To reduce its size, we can utilize a text compression algorithm. While compressing the string table, compressing the symbol table along with it might make sense, but using a compact encoding for the symbol table itself won't provide significant benefits.

Program headers

Program headers, while individually large (each Elf64_Phdr is 56 bytes) and no random access is needed, typically have a limited quantity within an executable or shared object. Consequently, their overall size contribution is relatively small. Light ELF maintains the existing format.

Section compression

Compressed sections face a challenge due to header overhead especially for ELFCLASS64.

The overhead and alignment padding limit the effectiveness when used with features like -ffunction-sections and -fdata-sections that generate many smaller sections. For example, I have found that the large Elf64_Chdr makes evaluating compressed .rela.* sections difficult. Light ELF addresses this challenge by introducing an header format of smaller footprint:

  • ch_type, ULEB128 encoded
  • ch_size, ULEB128 encoded
  • ch_addralign, ULEB128 encoded as log2 value

This approach allows Light ELF to represent the header information in just 3 bytes for smaller sections, compared to the 24 bytes required by the traditional format. T The content is no longer guaranteed to be word-aligned, a property that most compression libraries don't require anyway.

Furthermore, compressed sections with the SHF_ALLOC flag are allowed. Using them outside of relocatable files needs caution, though.

Experiments

I have developed a Clang/lld prototype that implements compact section header table and CREL.

`.o size`  |sht size   | build

-----------+-----------+----------------------------
 136012504 |  18284992 | -O3
 111583312 |  18284992 | -O3 -Wa,--crel
  97976973 |   4604341 | -O3 -Wa,--crel,--cshdr
2174179112 | 260281280 | -g
1763231672 | 260281280 | -g -Wa,--crel
1577187551 |  74234983 | -g -Wa,--crel,--cshdr

Light ELF: a thought experiment

By now, you might have realized that the post is about a joke. While bumping e_version and modifying Elf_Chdr might not be feasible, it's interesting to consider the possibilities of compact section headers and compressed symbol/string tables. Perhaps this can spark some interesting discussions!

I will update https://maskray.me/blog/2024-04-01-light-elf-exploring-potential-size-reduction as needed.

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