Resolves ifunc targets before `main()` runs in static libc
This enables static binaries to use ifunc-based dispatch during early
process startup, so optimized implementations can be selected based on
CPU features. Without this relocation step in startup, those targets are
not ready when program code begins executing.
This change:
- adds IRELATIVE relocation handling for x86_64, AArch64, ARMv7 and RISC-V,
- reads `AT_HWCAP` / `AT_HWCAP2` from auxv and passes them to resolvers
where required (notably AArch64),
- runs IRELATIVE processing after base-address discovery and before TLS
setup,
- adds integration tests for both the ifunc path and the no-ifunc path,
- Changed the load bias type for ptrdiff_t to intptr_t to align with
IRELATIVE handling, which uses intptr_t for load bias calculations.
In order to add Control-flow Enforcement Technology (CET) Shadow Stack
(SHSTK) support, we need to parse the `PT_GNU_PROPERTY` program header
and the corresponding section to evaluate if the binary being started
was compiled with the necessary support.
PS: This is my first PR to llvm-libc, I might have made obvious styling
mistakes so I'd appreciate any feedback or suggestions to improve it.
I have a prototype branch using this change to enable SHSTK support:
https://github.com/jakos-sec/llvm-project/tree/add-shstk-support
teardown_main_tls is called right before the main process exit to unmap
the TLS but that's unnecesary since the process is terminating anyway,
this just introduces extra system call. This can be potentially harmful
in case one of the other threads tries the access the memory that has
been unmapped. Other C library implementations don't unmap TLS either so
this makes LLVM libc's behavior more consistent.
Fixes#173409
Closes https://github.com/llvm/llvm-project/issues/153666
This patch introduces a new centralized AUXV (auxiliary vector) handling
mechanism for LLVM libc on Linux, replacing the previous scattered
implementation across multiple files.
## Key Changes:
### New Files:
- **libc/src/__support/OSUtil/linux/auxv.h**: New header library
providing
a clean interface for AUXV access with:
- `auxv::Entry` struct for AUXV entries (type and value)
- `auxv::Vector` class with iterator support for traversing AUXV
- `auxv::get()` function for retrieving specific AUXV values
- Thread-safe initialization with fallback mechanisms (prctl and
/proc/self/auxv)
### Modified Files:
1. **libc/src/__support/OSUtil/linux/CMakeLists.txt**:
- Added `auxv` header library declaration with proper dependencies:
- libc.hdr.fcntl_macros
- libc.src.__support.OSUtil.osutil
- libc.src.__support.common
- libc.src.__support.CPP.optional
- libc.src.__support.threads.callonce
2. **libc/config/linux/app.h**:
- Removed `AuxEntry` struct (moved to auxv.h as `auxv::Entry`)
- Removed `auxv_ptr` from `AppProperties` struct
- Simplified application properties structure
3. **libc/src/sys/auxv/linux/getauxval.cpp**:
- Completely refactored to use new auxv.h interface
- Removed ~200 lines of complex initialization code
- Simplified to just call `auxv::get()` function
- Removed dependencies to external symbols (mman, prctl, fcntl, read,
close, open)
4. **libc/src/sys/auxv/linux/CMakeLists.txt**:
- Updated dependencies to use new auxv header library
- Removed dependencies to external symbols (prctl, mman, fcntl, unistd,
etc.)
5. **libc/startup/linux/do_start.cpp**:
- Updated to use new `auxv::Vector` interface
- Changed from pointer-based to iterator-based AUXV traversal
- Updated field names (`aux_entry->id` → `aux_entry.type`,
`aux_entry->value` → `aux_entry.val`)
- Added call to `auxv::Vector::initialize_unsafe()` for early AUXV setup
6. **libc/startup/linux/CMakeLists.txt**:
- Added dependency on `libc.src.__support.OSUtil.linux.auxv`
The call chain to `Mutex:lock` can be polluted by stack protector. For
completely safe, let's postpone the main TLS tearing down to a separate
phase.
fix#108030
```
AppProperties app;
```
is marked as a weak symbol in header now. One can just use `&app !=
nullptr` to check if `app` is defined. There is no need to define it for
overlay mode.
For some reasons, we are using `-fpie`
(libc/cmake/modules/LLVMLibCObjectRules.cmake:31) without supporting it.
According to @lntue, some of the hermetic tests are broken without
proper PIE support. This patch implements basic relocations support for
PIE.
* separate initialization routines into _start and do_start for all
architectures.
* lift do_start as a separate object library to avoid code duplication.
* (addtionally) address the problem of building hermetic libc with
-fstack-pointer-*
The `crt1.o` is now a merged result of three components:
```
___
|___ x86_64
| |_______ start.cpp.o <- _start (loads process initial stack and aligns stack pointer)
| |_______ tls.cpp.o <- init_tls, cleanup_tls, set_thread_pointer (TLS related routines)
|___ do_start.cpp.o <- do_start (sets up global variables and invokes the main function)
```
