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feat: add __executable_start #1257

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feat: add __executable_start #1257

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83f681e
feat: add `__executable_start`
mati865 Nov 1, 2025
e3b33e0
move it to the right section
mati865 Nov 3, 2025
ea7eeb6
create more symbols
mati865 Nov 3, 2025
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4 changes: 4 additions & 0 deletions libwild/src/elf_writer.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -58,6 +58,7 @@ use crate::layout::SymbolCopyInfo;
use crate::layout::VersionDef;
use crate::layout::compute_allocations;
use crate::output_section_id;
use crate::output_section_id::EXECUTABLE_START;
use crate::output_section_id::OrderEvent;
use crate::output_section_id::OutputOrder;
use crate::output_section_id::OutputSectionId;
Expand Down Expand Up @@ -3124,6 +3125,9 @@ fn write_internal_symbols(
// __global_pointer$ = MIN(__SDATA_BEGIN__ + 0x800, MAX(__DATA_BEGIN__ + 0x800, __BSS_END__ - 0x800));
if symbol_name.bytes() == GLOBAL_POINTER_SYMBOL_NAME.as_bytes() {
address += RISCV_TLS_DTV_OFFSET;
} else if symbol_name.bytes() == EXECUTABLE_START.as_bytes() {
let text_layout = layout.section_layouts.get(output_section_id::TEXT);
address = text_layout.mem_offset;
}

let entry = symbol_writer
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2 changes: 2 additions & 0 deletions libwild/src/output_section_id.rs
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Expand Up @@ -399,6 +399,7 @@ const DEFAULT_DEFS: BuiltInSectionDetails = BuiltInSectionDetails {
target_segment_type: None,
};

pub(crate) const EXECUTABLE_START: &str = "__executable_start";
const SECTION_DEFINITIONS: [BuiltInSectionDetails; NUM_BUILT_IN_SECTIONS] = [
// A section into which we write headers.
BuiltInSectionDetails {
Expand Down Expand Up @@ -636,6 +637,7 @@ const SECTION_DEFINITIONS: [BuiltInSectionDetails; NUM_BUILT_IN_SECTIONS] = [
kind: SectionKind::Primary(SectionName(TEXT_SECTION_NAME)),
ty: sht::PROGBITS,
section_flags: shf::ALLOC.with(shf::EXECINSTR),
start_symbol_name: Some(EXECUTABLE_START),
..DEFAULT_DEFS
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@mati865 mati865 Nov 3, 2025

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This is wrong because that symbols should be emitted only for (some kinds of?) executables. Probably should be created inside Prelude::new.

Similarly, Wild outputs __ehdr_start even when other linkers don't, and it also misses __dso_handle symbol.

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@davidlattimore davidlattimore Nov 3, 2025

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I haven't looked into this too much, but a couple of things that you were probably aware of, but just in case... some of these symbols are PROVIDE in the GNU ld linker script, which AFAIK means that the linker script should only define the symbol if it's referenced and not defined by anything input objects. We don't yet support PROVIDE semantics, although it might be being worked on - #1097.

Some flags can affect the built-in linker script that GNU ld uses. You can see what changes for some particular flag with a command like meld <(ld --verbose) <(ld --pie --verbose).

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@mati865 mati865 Nov 4, 2025

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Thanks, I didn't see your comment when I was writing the other one (GitHub didn't refresh automatically). Well, I was unaware but figured that out eventually.

},
BuiltInSectionDetails {
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