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musl - an implementation of the standard library for Linux-based systems
e31c8c2d79
The TOC pointer is constant within a single dso, but needs to be saved and restored around cross-dso calls. The PLT stub saves it to the caller's stack frame, and the linker adds code to the caller to restore it. With a local call, as within a single dso or with static linking, this doesn't happen and the TOC pointer is always in r2. Therefore, setjmp/longjmp need to save/restore the TOC pointer from/to different locations depending on whether the call to setjmp was a local or non-local call. It is always safe for longjmp to restore to both r2 and the caller's stack. If the call to setjmp was local, and only r2 matters and the stack location will be ignored, but is required by the ABI to be reserved for the TOC pointer. If the call was non-local, then only the stack location matters, and whatever is restored into r2 will be clobbered anyway when the caller reloads r2 from the stack. A little extra care is required for sigsetjmp, because it uses setjmp internally. After the second return from this setjmp call, r2 will contain the caller's TOC pointer instead of libc's TOC pointer. We need to save and restore the correct libc pointer before we can tail call to __sigsetjmp_tail. |
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musl libc
musl, pronounced like the word "mussel", is an MIT-licensed
implementation of the standard C library targetting the Linux syscall
API, suitable for use in a wide range of deployment environments. musl
offers efficient static and dynamic linking support, lightweight code
and low runtime overhead, strong fail-safe guarantees under correct
usage, and correctness in the sense of standards conformance and
safety. musl is built on the principle that these goals are best
achieved through simple code that is easy to understand and maintain.
The 1.1 release series for musl features coverage for all interfaces
defined in ISO C99 and POSIX 2008 base, along with a number of
non-standardized interfaces for compatibility with Linux, BSD, and
glibc functionality.
For basic installation instructions, see the included INSTALL file.
Information on full musl-targeted compiler toolchains, system
bootstrapping, and Linux distributions built on musl can be found on
the project website:
http://www.musl-libc.org/