PR 34165 aarch64 assertion fail and segv
Fixes an assertion failure and segfault in aarch64 ELF linking related to weak undefined TLS symbols.
This patch resolves an assertion failure and subsequent segmentation fault in binutils’ AArch64 ELF linker. The issue arises when weak undefined TLS symbols are used, as their addresses are tied to the thread pointer. The fix modifies the dtpoff_base and tpoff_base functions to return 0 instead of asserting that tls_sec is non-NULL, thus preventing the crash and resulting in a zero .got entry for these symbols, consistent with x86 behavior.
- proposer
Applies a fix for PR34165, addressing a segfault and assertion failure in aarch64 ELF linking related to weak undefined TLS symbols.
“I'm going to apply this as a fix for pr34165. It fixes the segv and results in a zero .got entry for the weak undefined tls syms, the same as x86. It seems to me that weak undefined and tls don't work together very well, as the address of such a sym includes the thread pointer. So I don't think there is a reliable machine independent way to test whether the sym is defined.”
In Details
This patch addresses a segfault and assertion failure in the AArch64 ELF linker backend of binutils. The problem occurs when weak, undefined TLS symbols are encountered. The linker was asserting that the TLS section (tls_sec) must be non-NULL, but in cases involving weak undefined TLS symbols, tls_sec can legitimately be NULL. This patch changes the behavior from an assertion to returning 0, which correctly handles these symbols and produces a zero .got entry, aligning behavior with the x86 backend.
- TLS
- Thread-Local Storage, a mechanism that allows variables to have different values for each thread in a multi-threaded program.
- weak undefined symbol
- A symbol that is declared but not defined, and has weak linkage. This means a definition may be provided elsewhere, but if not, the program can still link and potentially run, often with default or zero values.
- AArch64
- The 64-bit execution state of the ARM architecture.
- ELF
- Executable and Linkable Format, a standard file format for executables, object code, shared libraries, and core dumps on many Unix-like systems.
- segfault
- A segmentation fault, an error that occurs when a program accesses a memory location that it's not allowed to access.
- .got
- Global Offset Table, a section in ELF executables and dynamic libraries used to store pointers to dynamically resolved external function or variable references.