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feat: support the DEFINED command in linker scripts #2181

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feat: support the DEFINED command in linker scripts #2181

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582ce2d
feat: support the DEFINED command in linker scripts
vishruth-thimmaiah Jul 5, 2026
ad2d867
tests: check for weak symbols
vishruth-thimmaiah Jul 8, 2026
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4 changes: 2 additions & 2 deletions LINKER_SCRIPT_SUPPORT.md
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Expand Up @@ -82,7 +82,7 @@ end lists the features required to link the Linux kernel.
| `MIN(a, b)` | ✅ | |
| `MAX(a, b)` | ✅ | |
| Ternary operator (`condition ? a : b`) | ✅ | |
| `DEFINED(sym)` | 📅 | |
| `DEFINED(sym)` | ✅ | |
| `SIZEOF_HEADERS` | ✅ | |
| `SEGMENT_START(segment, default)` | ✅ | Supports `"text"`, `"data"`, `"bss"`, `"rodata"`; returns `-Ttext`/`-Tdata`/`-Tbss` override if provided, otherwise `default`; unknown segment names always return `default` |

Expand Down Expand Up @@ -122,6 +122,6 @@ see at a glance what remains before Wild can link the kernel.
| `CONSTRUCTORS` command | 📅 | |
| `PHDRS` command for explicit program header definition | 🧪 | The FILEHDR and PHDRS keywords aren't yet supported. |
| Ternary operator (`condition ? a : b`) | ✅ | |
| `DEFINED(sym)` function | 📅 | |
| `DEFINED(sym)` function | ✅ | |
| `SIZEOF_HEADERS` built-in symbol | ✅ | |
| `/DISCARD/` command | ✅ | |
3 changes: 3 additions & 0 deletions libwild/src/expression_eval.rs
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Expand Up @@ -213,6 +213,9 @@ pub(crate) fn evaluate_expression<'data, P: Platform>(
eval!(if_false)
}
}
Expression::Defined(name) => Ok(symbol_db
.get_unversioned(&UnversionedSymbolName::prehashed(name))

@davidlattimore davidlattimore Jul 6, 2026

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I'm not sure that this will do the correct thing if there are objects that reference, but don't define the symbol. Can you add a test for that? i.e. have an object that has a weak reference to the symbol.

.map_or(0, |_| 1)),
}
}

Expand Down
11 changes: 9 additions & 2 deletions libwild/src/linker_script.rs
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Expand Up @@ -170,7 +170,8 @@ pub(crate) struct Phdr<'a> {
/// - Bitwise: &, |, ^, ~, <<, >>
/// - Logical: &&, ||
/// - Unary: -, !, ~
/// - Functions: SIZEOF, ALIGNOF, LENGTH, ORIGIN, ADDR, LOADADDR, ALIGN, MIN, MAX, SEGMENT_START
/// - Functions: SIZEOF, ALIGNOF, LENGTH, ORIGIN, ADDR, LOADADDR, ALIGN, MIN, MAX, SEGMENT_START,
/// DEFINED
/// - Numbers (hex/decimal), symbols, location counter (.)
/// - Parentheses for grouping
/// - Ternary operator (? :)
Expand Down Expand Up @@ -229,6 +230,7 @@ pub(crate) enum Expression<'a> {
Box<Expression<'a>>,
Box<Expression<'a>>,
),
Defined(&'a [u8]),
}

#[derive(Debug, PartialEq, Eq, Clone)]
Expand Down Expand Up @@ -261,7 +263,8 @@ impl<'a> Expression<'a> {
| Expression::Addr(_)
| Expression::Loadaddr(_)
| Expression::Symbol(_)
| Expression::SizeofHeaders => {}
| Expression::SizeofHeaders
| Expression::Defined(_) => {}
Expression::Add(l, r)
| Expression::Subtract(l, r)
| Expression::Multiply(l, r)
Expand Down Expand Up @@ -926,6 +929,10 @@ fn parse_identifier_or_function<'a>(input: &mut &'a BStr) -> winnow::Result<Expr
Box::new(default_expr),
))
}
b"DEFINED" => {
let symbol = parse_function_arg.parse_next(input)?;
Ok(Expression::Defined(symbol))
}
_ => Err(ContextError::default()),
}
} else if ident == b"SIZEOF_HEADERS" {
Expand Down
6 changes: 6 additions & 0 deletions wild/tests/sources/elf/linker-script-assert-pass/linker-script-assert-pass.c
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Expand Up @@ -2,3 +2,9 @@
//#RunEnabled:false
//#DiffEnabled:false
void _start() {}

int symbol4 = 0x4000;
int symbol6 __attribute__((weak));

@davidlattimore davidlattimore Jul 8, 2026

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Note that declaring a variable doesn't create a reference to it - you need to actually have some code that uses the variable for it to end up in the symbol table.

@davidlattimore davidlattimore Jul 8, 2026

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You can verify this by running readelf -Ws on the object file to see what undefined symbols are listed.

int symbol7 __attribute__((weak)) = 0x7000;
extern int symbol8 __attribute__((weak));
extern int symbol9;
13 changes: 12 additions & 1 deletion wild/tests/sources/elf/linker-script-assert-pass/linker-script-assert-pass.ld
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@@ -1,7 +1,10 @@
ENTRY(_start)

SECTIONS {
.text : { *(.text*) }
.text : {
*(.text*)
symbol3 = 0x3000;
}
.data : { *(.data*) }
.bss : { *(.bss*) }
}
Expand All @@ -27,3 +30,11 @@ ASSERT(symbol1 + symbol2 != symbol1 - symbol2, "symbol1 + symbol2 must not be sy
ASSERT(0xffffffff81000000 == (0xffffffff80000000 + (((symbol1 * 0x1000) + (symbol2 * 0x100 - 1)) & ~(0x200000 - 1))), "Incorrect Value");
ASSERT((5 > 4 ? !1 ? 3 : 5 : 4) == 5, "5 > 4 ? !1 ? 3 : 5 : 4 must be 5");
ASSERT((symbol1 > symbol2 ? symbol1 : symbol2) == MAX(symbol1, symbol2), "Expected MAX(symbol1, symbol2)");
ASSERT(DEFINED(symbol1), "symbol1 must be defined");
ASSERT(DEFINED(symbol3), "symbol3 must be defined");
ASSERT(DEFINED(symbol4), "symbol4 must be defined");
ASSERT(!DEFINED(symbol5), "symbol5 should not be defined");
ASSERT(DEFINED(symbol6), "symbol6 must be defined");
ASSERT(DEFINED(symbol7), "symbol7 must be defined");
ASSERT(!DEFINED(symbol8), "symbol8 should not be defined");
ASSERT(!DEFINED(symbol9), "symbol9 should not be defined");
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