From: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx> This commit adds the initial implementation of nolibc `sigaction()` function. Currently, this implementation is only available on the x86-64 arch. `sigaction()` needs an architecture-dependent "signal trampoline" function that invokes __rt_sigreturn syscall to resume the process after a signal gets handled. The "signal trampoline" function is called `__restore_rt` in this implementation. The naming `__restore_rt` is important for GDB. It also has to be given a special optimization attribute "omit-frame-pointer" to prevent the compiler from creating a stack frame that makes the `%rsp` value no longer points to the `struct rt_sigframe` that the kernel constructed. Link: https://lore.kernel.org/lkml/20221228133513.GA7457@xxxxxx Suggested-by: Willy Tarreau <w@xxxxxx> Signed-off-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx> --- Side note: This has been tested on x86-64 arch and `__restore_rt` generates the correct code. The `__restore_rt` codegen correctness on other architectures need to be evaluated as well. If it can't generate the correct code, it has to be written in inline Assembly. tools/include/nolibc/sys.h | 72 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 72 insertions(+) diff --git a/tools/include/nolibc/sys.h b/tools/include/nolibc/sys.h index acf7cf438010..7d594155e77f 100644 --- a/tools/include/nolibc/sys.h +++ b/tools/include/nolibc/sys.h @@ -1047,6 +1047,78 @@ pid_t setsid(void) return ret; } +typedef void (*sighandler_t)(int sig); + +/* + * int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact); + */ + +static __attribute__((unused)) +int sys_sigaction(int signum, const struct sigaction *act, + struct sigaction *oldact) +{ + return my_syscall4(__NR_rt_sigaction, signum, act, oldact, + sizeof(sigset_t)); +} + +__attribute__((weak,unused,noreturn,optimize("omit-frame-pointer"),section(".text.__restore_rt"))) +void __restore_rt(void) +{ + my_syscall0(__NR_rt_sigreturn); + __builtin_unreachable(); +} + +static __attribute__((unused)) +int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact) +{ + struct sigaction act2 = *act; + int ret; + + /* + * On Linux x86-64, libc's sigaction() always sets the + * @act->sa_restorer when the caller passes a NULL. + * + * @act->sa_restorer is an arch-specific function used + * as a "signal trampoline". + * + * @act->sa_handler is a signal handler provided by the + * user. + * + * When the handled signal is caught, the %rip jumps to + * @act->sa_handler with user stack already set by the + * kernel as below: + * + * |--------------------| + * %rsp -> | act->sa_restorer | (return address) + * |--------------------| + * | struct rt_sigframe | (process context info) + * | | + * | | + * .................... + * + * Once this signal handler executes the "ret" instruction, + * the %rip jumps to @act->sa_restorer. The sa_restorer + * function has to invoke the __rt_sigreturn syscall with + * %rsp pointing to the `struct rt_sigframe` that the kernel + * constructed previously to resume the process. + * + * `struct rt_sigframe` contains the registers' value before + * the signal is caught. + * + */ + if (!act2.sa_restorer) { + act2.sa_flags |= SA_RESTORER; + act2.sa_restorer = __restore_rt; + } + + ret = sys_sigaction(signum, &act2, oldact); + if (ret < 0) { + SET_ERRNO(-ret); + ret = -1; + } + return ret; +} + /* * int stat(const char *path, struct stat *buf); -- Ammar Faizi
