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LibELF: Add support for IFUNCs

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IFUNC is a GNU extension to the ELF standard that allows a function to
have multiple implementations. A resolver function has to be called at
load time to choose the right one to use. The PLT will contain the entry
to the resolved function, so branching and more indirect jumps can be
avoided at run-time.

This mechanism is usually used when a routine can be made faster using
CPU features that are available in only some models, and a fallback
implementation has to exist for others.

We will use this feature to have two separate memset implementations for
CPUs with and without ERMS (Enhanced REP MOVSB/STOSB) support.
This commit is contained in:
Daniel Bertalan
2022-03-13 08:49:28 +01:00
committed by Andreas Kling
parent 4d5965bd2c
commit 08c459e495
6 changed files with 56 additions and 10 deletions
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5
Userland/Libraries/LibELF/DynamicObject.cpp
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@@ -471,7 +471,7 @@ auto DynamicObject::lookup_symbol(HashSymbol const& symbol) const -> Optional<Sy
auto symbol_result = result.value();
if (symbol_result.is_undefined())
return {};
return SymbolLookupResult { symbol_result.value(), symbol_result.size(), symbol_result.address(), symbol_result.bind(), this };
return SymbolLookupResult { symbol_result.value(), symbol_result.size(), symbol_result.address(), symbol_result.bind(), symbol_result.type(), this };
}
NonnullRefPtr<DynamicObject> DynamicObject::create(String const& filename, VirtualAddress base_address, VirtualAddress dynamic_section_address)
@@ -495,6 +495,9 @@ VirtualAddress DynamicObject::patch_plt_entry(u32 relocation_offset)
auto result = DynamicLoader::lookup_symbol(symbol);
if (result.has_value()) {
symbol_location = result.value().address;
if (result.value().type == STT_GNU_IFUNC)
symbol_location = VirtualAddress { reinterpret_cast<IfuncResolver>(symbol_location.get())() };
} else if (symbol.bind() != STB_WEAK) {
dbgln("did not find symbol while doing relocations for library {}: {}", m_filename, symbol.name());
VERIFY_NOT_REACHED();