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write/macho: ensure Mach-O subsections are not zero size
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For Mach-O, `add_symbol_data` now ensures that the symbol size
is at least 1 when subsections via symbols are enabled.
This change was made to support linking with ld-prime. It is also
unclear how this previously managed to work with ld64.

`write::Object::add_subsection` no longer enables subsections
via symbols for Mach-O. Use `set_subsections_via_symbols` instead.
This change was made because Mach-O subsections are all or nothing,
so this decision must be made before any symbols are added.

`write::Object::add_subsection` no longer adds data to the subsection.
This change was made because it was done with `append_section_data`,
but this is often not the correct way to add data to the subsection.
Usually `add_symbol_data` is a better choice.
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@philipc
philipc committed May 4, 2024
1 parent 1f3e12c commit 6b4fc60
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Showing 5 changed files with 54 additions and 49 deletions.
4 changes: 2 additions & 2 deletions crates/examples/src/bin/simple_write.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -66,8 +66,8 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
main_data.extend_from_slice(&[0xc3]);

// Add the main function in its own subsection (equivalent to -ffunction-sections).
let (main_section, main_offset) =
obj.add_subsection(StandardSection::Text, b"main", &main_data, 1);
let main_section = obj.add_subsection(StandardSection::Text, b"main");
let main_offset = obj.append_section_data(main_section, &main_data, 1);
// Add a globally visible symbol for the main function.
obj.add_symbol(Symbol {
name: b"main".into(),
Expand Down
15 changes: 4 additions & 11 deletions src/write/macho.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -66,16 +66,6 @@ impl<'a> Object<'a> {

// Private methods.
impl<'a> Object<'a> {
pub(crate) fn macho_set_subsections_via_symbols(&mut self) {
let flags = match self.flags {
FileFlags::MachO { flags } => flags,
_ => 0,
};
self.flags = FileFlags::MachO {
flags: flags | macho::MH_SUBSECTIONS_VIA_SYMBOLS,
};
}

pub(crate) fn macho_segment_name(&self, segment: StandardSegment) -> &'static [u8] {
match segment {
StandardSegment::Text => &b"__TEXT"[..],
Expand Down Expand Up @@ -570,10 +560,13 @@ impl<'a> Object<'a> {
cpusubtype = cpu_subtype;
}

let flags = match self.flags {
let mut flags = match self.flags {
FileFlags::MachO { flags } => flags,
_ => 0,
};
if self.macho_subsections_via_symbols {
flags |= macho::MH_SUBSECTIONS_VIA_SYMBOLS;
}
macho.write_mach_header(
buffer,
MachHeader {
Expand Down
64 changes: 38 additions & 26 deletions src/write/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -90,6 +90,9 @@ pub struct Object<'a> {
macho_cpu_subtype: Option<u32>,
#[cfg(feature = "macho")]
macho_build_version: Option<MachOBuildVersion>,
/// Mach-O MH_SUBSECTIONS_VIA_SYMBOLS flag. Only ever set if format is Mach-O.
#[cfg(feature = "macho")]
macho_subsections_via_symbols: bool,
}

impl<'a> Object<'a> {
Expand All @@ -115,6 +118,8 @@ impl<'a> Object<'a> {
macho_cpu_subtype: None,
#[cfg(feature = "macho")]
macho_build_version: None,
#[cfg(feature = "macho")]
macho_subsections_via_symbols: false,
}
}

Expand Down Expand Up @@ -273,41 +278,34 @@ impl<'a> Object<'a> {

/// Add a subsection. Returns the `SectionId` and section offset of the data.
///
/// Must not be called for sections that contain uninitialized data.
/// `align` must be a power of two.
pub fn add_subsection(
&mut self,
section: StandardSection,
name: &[u8],
data: &[u8],
align: u64,
) -> (SectionId, u64) {
let section_id = if self.has_subsections_via_symbols() {
self.set_subsections_via_symbols();
/// For Mach-O, this does not create a subsection, and instead uses the
/// section from [`Self::section_id`]. Use [`Self::set_subsections_via_symbols`]
/// to enable subsections via symbols.
pub fn add_subsection(&mut self, section: StandardSection, name: &[u8]) -> SectionId {
if self.has_subsections_via_symbols() {
self.section_id(section)
} else {
let (segment, name, kind, flags) = self.subsection_info(section, name);
let id = self.add_section(segment.to_vec(), name, kind);
self.section_mut(id).flags = flags;
id
};
let offset = self.append_section_data(section_id, data, align);
(section_id, offset)
}
}

fn has_subsections_via_symbols(&self) -> bool {
match self.format {
BinaryFormat::Coff | BinaryFormat::Elf | BinaryFormat::Xcoff => false,
BinaryFormat::MachO => true,
_ => unimplemented!(),
}
self.format == BinaryFormat::MachO
}

fn set_subsections_via_symbols(&mut self) {
match self.format {
#[cfg(feature = "macho")]
BinaryFormat::MachO => self.macho_set_subsections_via_symbols(),
_ => unimplemented!(),
/// Enable subsections via symbols if supported.
///
/// This should be called before adding any subsections or symbols.
///
/// For Mach-O, this sets the `MH_SUBSECTIONS_VIA_SYMBOLS` flag.
/// For other formats, this does nothing.
pub fn set_subsections_via_symbols(&mut self) {
#[cfg(feature = "macho")]
if self.format == BinaryFormat::MachO {
self.macho_subsections_via_symbols = true;
}
}

Expand Down Expand Up @@ -480,6 +478,9 @@ impl<'a> Object<'a> {
/// For Mach-O, this also creates a `__thread_vars` entry for TLS symbols, and the
/// symbol will indirectly point to the added data via the `__thread_vars` entry.
///
/// For Mach-O, if [`Self::set_subsections_via_symbols`] is enabled, this will
/// automatically ensure the data size is at least 1.
///
/// Returns the section offset of the data.
///
/// Must not be called for sections that contain uninitialized data.
Expand All @@ -488,9 +489,13 @@ impl<'a> Object<'a> {
&mut self,
symbol_id: SymbolId,
section: SectionId,
data: &[u8],
mut data: &[u8],
align: u64,
) -> u64 {
#[cfg(feature = "macho")]
if data.is_empty() && self.macho_subsections_via_symbols {
data = &[0];
}
let offset = self.append_section_data(section, data, align);
self.set_symbol_data(symbol_id, section, offset, data.len() as u64);
offset
Expand All @@ -501,6 +506,9 @@ impl<'a> Object<'a> {
/// For Mach-O, this also creates a `__thread_vars` entry for TLS symbols, and the
/// symbol will indirectly point to the added data via the `__thread_vars` entry.
///
/// For Mach-O, if [`Self::set_subsections_via_symbols`] is enabled, this will
/// automatically ensure the data size is at least 1.
///
/// Returns the section offset of the data.
///
/// Must not be called for sections that contain initialized data.
Expand All @@ -509,9 +517,13 @@ impl<'a> Object<'a> {
&mut self,
symbol_id: SymbolId,
section: SectionId,
size: u64,
mut size: u64,
align: u64,
) -> u64 {
#[cfg(feature = "macho")]
if size == 0 && self.macho_subsections_via_symbols {
size = 1;
}
let offset = self.append_section_bss(section, size, align);
self.set_symbol_data(symbol_id, section, offset, size);
offset
Expand Down
16 changes: 8 additions & 8 deletions tests/round_trip/comdat.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,11 +13,11 @@ fn coff_x86_64_comdat() {
let mut object =
write::Object::new(BinaryFormat::Coff, Architecture::X86_64, Endianness::Little);

let (section1, offset) =
object.add_subsection(write::StandardSection::Text, b"s1", &[0, 1, 2, 3], 4);
let section1 = object.add_subsection(write::StandardSection::Text, b"s1");
let offset = object.append_section_data(section1, &[0, 1, 2, 3], 4);
object.section_symbol(section1);
let (section2, _) =
object.add_subsection(write::StandardSection::Data, b"s1", &[0, 1, 2, 3], 4);
let section2 = object.add_subsection(write::StandardSection::Data, b"s1");
object.append_section_data(section2, &[0, 1, 2, 3], 4);
object.section_symbol(section2);

let symbol = object.add_symbol(write::Symbol {
Expand Down Expand Up @@ -132,10 +132,10 @@ fn elf_x86_64_comdat() {
let mut object =
write::Object::new(BinaryFormat::Elf, Architecture::X86_64, Endianness::Little);

let (section1, offset) =
object.add_subsection(write::StandardSection::Text, b"s1", &[0, 1, 2, 3], 4);
let (section2, _) =
object.add_subsection(write::StandardSection::Data, b"s1", &[0, 1, 2, 3], 4);
let section1 = object.add_subsection(write::StandardSection::Text, b"s1");
let offset = object.append_section_data(section1, &[0, 1, 2, 3], 4);
let section2 = object.add_subsection(write::StandardSection::Data, b"s1");
object.append_section_data(section2, &[0, 1, 2, 3], 4);

let symbol = object.add_symbol(write::Symbol {
name: b"s1".to_vec(),
Expand Down
4 changes: 2 additions & 2 deletions tests/round_trip/elf.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,8 +13,8 @@ fn symtab_shndx() {

for i in 0..0x10000 {
let name = format!("func{}", i).into_bytes();
let (section, offset) =
object.add_subsection(write::StandardSection::Text, &name, &[0xcc], 1);
let section = object.add_subsection(write::StandardSection::Text, &name);
let offset = object.append_section_data(section, &[0xcc], 1);
object.add_symbol(write::Symbol {
name,
value: offset,
Expand Down

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