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[ELF] Check COMMON symbols for PROVIDE and don't redefine COMMON symbols edata/end/etext

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In GNU ld, the definition precedence is: regular symbol assignment > relocatable object definition > `PROVIDE` symbol assignment.

GNU ld's internal linker scripts define the non-reserved (by C and C++)
edata/end/etext with `PROVIDE` so the relocatable object definition takes
precedence. This makes sense because `int end;` is valid.

We currently redefine such symbols if they are COMMON, but not if they are
regular definitions, so `int end;` with -fcommon is essentially a UB in ld.lld.
Fix this (also improve consistency and match GNU ld) by using the
`isDefined` code path for `isCommon`. In GNU ld, reserved identifiers like
`__ehdr_start` do not use `PROVIDE`, while we treat them all as `PROVIDE`, this
seems fine.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D120389
This commit is contained in:
Fangrui Song
2022-02-23 10:15:42 -08:00
parent b3a778fb5e
commit fc0aa8424c
3 changed files with 53 additions and 5 deletions
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2
lld/ELF/LinkerScript.cpp
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@@ -203,7 +203,7 @@ static bool shouldDefineSym(SymbolAssignment *cmd) {
// If a symbol was in PROVIDE(), we need to define it only
// when it is a referenced undefined symbol.
Symbol *b = symtab->find(cmd->name);
if (b && !b->isDefined())
if (b && !b->isDefined() && !b->isCommon())
return true;
return false;
}