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Preserve empty REL section slots dropped by mwld #145

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Preserve empty REL section slots dropped by mwld #145

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70 changes: 41 additions & 29 deletions src/cmd/rel.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -42,7 +42,8 @@ use crate::{
path::native_path,
rel::{
PERMITTED_SECTIONS, RelHeader, RelReloc, RelSectionHeader, RelWriteInfo,
print_relocations, process_rel, process_rel_header, process_rel_sections, write_rel,
is_permitted_section, print_relocations, process_rel, process_rel_header,
process_rel_sections, write_rel,
},
},
vfs::open_file,
Expand Down Expand Up @@ -140,34 +141,27 @@ fn match_section_index(
section_index: SectionIndex,
rel_sections: &[RelSectionHeader],
) -> Result<usize> {
let (_, _) = (obj, rel_sections);
Ok(section_index.0)
// TODO
// rel_sections
// .iter()
// .enumerate()
// .filter(|(_, s)| s.size() > 0)
// .zip(obj.sections().filter(|s| s.size() > 0))
// .find_map(
// |((rel_section_index, _), obj_section)| {
// if obj_section.index() == section_index {
// Some(rel_section_index)
// } else {
// None
// }
// },
// )
// .ok_or_else(|| {
// anyhow!(
// "Failed to find matching section index for {} ({}), REL section count: {}",
// obj.section_by_index(section_index)
// .ok()
// .and_then(|s| s.name().ok().map(|s| s.to_string()))
// .unwrap_or("[invalid]".to_string()),
// section_index.0,
// rel_sections.len()
// )
// })
rel_sections
.iter()
.enumerate()
.filter(|(_, s)| s.size() > 0)
.zip(obj.sections().filter(|s| is_permitted_section(s) && s.size() > 0))
.find_map(
|((rel_section_index, _), obj_section)| {
if obj_section.index() == section_index { Some(rel_section_index) } else { None }
},
)
.ok_or_else(|| {
anyhow!(
"Failed to find matching section index for {} ({}), REL section count: {}",
obj.section_by_index(section_index)
.ok()
.and_then(|s| s.name().ok().map(|s| s.to_string()))
.unwrap_or("[invalid]".to_string()),
section_index.0,
rel_sections.len()
)
})
}

fn load_rel(module_config: &ModuleConfig, object_base: &ObjectBase) -> Result<RelInfo> {
Expand Down Expand Up @@ -384,6 +378,7 @@ fn make(args: MakeArgs) -> Result<()> {
quiet: args.no_warn,
section_align: None,
section_exec: None,
section_index_map: None,
};
if let Some((header, section_headers, section_defs)) =
existing_headers.get(&module_info.module_id)
Expand All @@ -399,6 +394,23 @@ fn make(args: MakeArgs) -> Result<()> {
.map(|defs| defs.iter().map(|def| def.align).collect())
.unwrap_or_default();
info.section_exec = Some(section_headers.iter().map(|s| s.exec()).collect());
// Where each ELF section lands in the original's section table. They
// differ whenever the original reserved a slot that mwld dropped for
// being empty, which shifts everything after it by one.
let mut map = vec![0usize; module_info.file.sections().count()];
for ((rel_idx, _), obj_section) in
section_headers.iter().enumerate().filter(|(_, s)| s.size() > 0).zip(
module_info.file.sections().filter(|s| is_permitted_section(s) && s.size() > 0),
)
{
map[obj_section.index().0] = rel_idx;
}
// Only carry the map when it actually differs from the ELF's own
// indices. Projects whose RELs have no dropped slot then take the
// exact path they took before.
if map.iter().enumerate().any(|(elf_idx, &rel_idx)| elf_idx != rel_idx) {
info.section_index_map = Some(map);
}
}
let rel_path = module_info.path.with_extension("rel");
let mut w = buf_writer(&rel_path)?;
Expand Down
54 changes: 49 additions & 5 deletions src/util/rel.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -691,6 +691,11 @@ pub struct RelWriteInfo {
/// This is used to match empty sections: mwld will emit them with
/// NULL type, but the original REL may have them marked executable.
pub section_exec: Option<Vec<bool>>,
/// Map from ELF section index to REL section index.
/// Some games reserve an empty section slot that mwld drops during the
/// partial link, because nothing lands in it. Without this the rebuilt
/// REL packs its sections one index lower than the original.
pub section_index_map: Option<Vec<usize>>,
}

pub const PERMITTED_SECTIONS: [&str; 7] =
Expand Down Expand Up @@ -968,21 +973,24 @@ where
header.fix_size = Some(offset);
}

let map_section = |idx: usize| -> u8 {
info.section_index_map.as_ref().and_then(|m| m.get(idx).copied()).unwrap_or(idx) as u8
};
for symbol in file.symbols().filter(|s| s.is_definition()) {
let Some(symbol_section) = symbol.section_index() else {
continue;
};
match symbol.name() {
Ok("_prolog") => {
header.prolog_section = symbol_section.0 as u8;
header.prolog_section = map_section(symbol_section.0);
header.prolog_offset = symbol.address() as u32;
}
Ok("_epilog") => {
header.epilog_section = symbol_section.0 as u8;
header.epilog_section = map_section(symbol_section.0);
header.epilog_offset = symbol.address() as u32;
}
Ok("_unresolved") => {
header.unresolved_section = symbol_section.0 as u8;
header.unresolved_section = map_section(symbol_section.0);
header.unresolved_offset = symbol.address() as u32;
}
_ => {}
Expand All @@ -993,9 +1001,45 @@ where
ensure!(w.stream_position()? as u32 == header.section_info_offset);
let mut current_data_offset = section_data_offset;
let mut permitted_section_idx = 0;
// Reverse of section_index_map: for each REL slot, the ELF section that
// belongs in it. Slots with no ELF section are the placeholders mwld drops.
let rel_to_elf: Option<Vec<Option<usize>>> = info.section_index_map.as_ref().map(|map| {
let mut rev = vec![None; num_sections as usize];
for (elf_idx, &rel_idx) in map.iter().enumerate() {
if rel_idx < rev.len() {
rev[rel_idx] = Some(elf_idx);
}
}
rev
});
for section_index in 0..num_sections {
let Ok(section) = file.section_by_index(object::SectionIndex(section_index as usize))
else {
let elf_index = match &rel_to_elf {
Some(rev) => match rev[section_index as usize] {
Some(i) => i,
None => {
// No ELF section belongs in this slot. Two cases: a slot the
// original reserved between real sections, which mwld drops
// for being empty and which points at where its data would
// start; or a trailing slot past the last real section,
// which is left at zero.
let last_real = rev.iter().rposition(|e| e.is_some()).unwrap_or(0);
let (offset, exec) = if (section_index as usize) < last_real {
let exec = info
.section_exec
.as_ref()
.and_then(|m| m.get(section_index as usize).copied())
.unwrap_or(false);
(current_data_offset, exec)
} else {
(0, false)
};
RelSectionHeader::new(offset, 0, exec).to_writer(w, Endian::Big)?;
continue;
}
},
None => section_index as usize,
};
let Ok(section) = file.section_by_index(object::SectionIndex(elf_index)) else {
RelSectionHeader::new(0, 0, false).to_writer(w, Endian::Big)?;
continue;
};
Expand Down