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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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46
Userland/Libraries/LibELF/DynamicLoader.cpp
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@ -2,6 +2,7 @@
* Copyright (c) 2019-2020, Andrew Kaster <akaster@serenityos.org>
* Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
* Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
* Copyright (c) 2022, Daniel Bertalan <dani@danielbertalan.dev>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
@ -177,6 +178,15 @@ bool DynamicLoader::load_stage_2(unsigned flags)
return false;
}
}
} else {
// .text needs to be executable while we process relocations because it might contain IFUNC resolvers.
// We don't allow IFUNC resolvers in objects with textrels.
for (auto& text_segment : m_text_segments) {
if (mprotect(text_segment.address().as_ptr(), text_segment.size(), PROT_READ | PROT_EXEC) < 0) {
perror("mprotect .text: PROT_READ | PROT_EXEC");
return false;
}
}
}
do_main_relocations();
return true;
@ -210,9 +220,12 @@ Result<NonnullRefPtr<DynamicObject>, DlErrorMessage> DynamicLoader::load_stage_3
setup_plt_trampoline();
}
for (auto& text_segment : m_text_segments) {
if (mprotect(text_segment.address().as_ptr(), text_segment.size(), PROT_READ | PROT_EXEC) < 0) {
return DlErrorMessage { String::formatted("mprotect .text: PROT_READ | PROT_EXEC: {}", strerror(errno)) };
if (m_dynamic_object->has_text_relocations()) {
// If we don't have textrels, .text has already been made executable by this point in load_stage_2.
for (auto& text_segment : m_text_segments) {
if (mprotect(text_segment.address().as_ptr(), text_segment.size(), PROT_READ | PROT_EXEC) < 0) {
return DlErrorMessage { String::formatted("mprotect .text: PROT_READ | PROT_EXEC: {}", strerror(errno)) };
}
}
}
@ -411,6 +424,10 @@ DynamicLoader::RelocationResult DynamicLoader::do_relocation(const ELF::DynamicO
else
patch_ptr = (FlatPtr*)(FlatPtr)relocation.offset();
auto call_ifunc_resolver = [](VirtualAddress address) {
return VirtualAddress { reinterpret_cast<DynamicObject::IfuncResolver>(address.get())() };
};
switch (relocation.type()) {
#if ARCH(I386)
case R_386_NONE:
@ -438,6 +455,8 @@ DynamicLoader::RelocationResult DynamicLoader::do_relocation(const ELF::DynamicO
*patch_ptr = symbol_address.get() + relocation.addend();
else
*patch_ptr += symbol_address.get();
if (res.value().type == STT_GNU_IFUNC)
*patch_ptr = call_ifunc_resolver(VirtualAddress { *patch_ptr }).get();
break;
}
#if ARCH(I386)
@ -467,8 +486,11 @@ DynamicLoader::RelocationResult DynamicLoader::do_relocation(const ELF::DynamicO
}
symbol_location = VirtualAddress { (FlatPtr)0 };
} else
} else {
symbol_location = res.value().address;
if (res.value().type == STT_GNU_IFUNC)
symbol_location = call_ifunc_resolver(symbol_location);
}
VERIFY(symbol_location != m_dynamic_object->base_address());
*patch_ptr = symbol_location.get();
break;
@ -500,6 +522,7 @@ DynamicLoader::RelocationResult DynamicLoader::do_relocation(const ELF::DynamicO
auto res = lookup_symbol(symbol);
if (!res.has_value())
break;
VERIFY(symbol.type() != STT_GNU_IFUNC);
symbol_value = res.value().value;
dynamic_object_of_symbol = res.value().dynamic_object;
} else {
@ -529,6 +552,19 @@ DynamicLoader::RelocationResult DynamicLoader::do_relocation(const ELF::DynamicO
}
break;
}
#if ARCH(I386)
case R_386_IRELATIVE: {
#else
case R_X86_64_IRELATIVE: {
#endif
VirtualAddress resolver;
if (relocation.addend_used())
resolver = m_dynamic_object->base_address().offset(relocation.addend());
else
resolver = m_dynamic_object->base_address().offset(*patch_ptr);
*patch_ptr = call_ifunc_resolver(resolver).get();
break;
}
default:
// Raise the alarm! Someone needs to implement this relocation type
dbgln("Found a new exciting relocation type {}", relocation.type());
@ -635,7 +671,7 @@ Optional<DynamicObject::SymbolLookupResult> DynamicLoader::lookup_symbol(const E
if (symbol.is_undefined() || symbol.bind() == STB_WEAK)
return DynamicLinker::lookup_global_symbol(symbol.name());
return DynamicObject::SymbolLookupResult { symbol.value(), symbol.size(), symbol.address(), symbol.bind(), &symbol.object() };
return DynamicObject::SymbolLookupResult { symbol.value(), symbol.size(), symbol.address(), symbol.bind(), symbol.type(), &symbol.object() };
}
} // end namespace ELF