From: Ammar Faizi ammarfaizi2@gnuweeb.org
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 the __rt_sigreturn syscall to resume the process after a signal gets handled.
On Linux x86-64, the "signal trampoline" function has to be written in inline Assembly to prevent the compiler from controlling the `%rsp` (e.g., with `-fno-omit-frame-pointer`, every function has a `pushq %rbp` that makes the `%rsp` no longer point to `struct rt_sigframe`).
The "signal trampoline" function is called `__arch_restore_rt` in this implementation.
Signed-off-by: Ammar Faizi ammarfaizi2@gnuweeb.org --- tools/include/nolibc/arch-x86_64.h | 12 +++++ tools/include/nolibc/sys.h | 80 ++++++++++++++++++++++++++++++ 2 files changed, 92 insertions(+)
diff --git a/tools/include/nolibc/arch-x86_64.h b/tools/include/nolibc/arch-x86_64.h index 0e1e9eb8545d..b6470943e836 100644 --- a/tools/include/nolibc/arch-x86_64.h +++ b/tools/include/nolibc/arch-x86_64.h @@ -212,4 +212,16 @@ __asm__ (".section .text\n" "hlt\n" // ensure it does not return "");
+void __arch_restore_rt(void); + +__asm__ ( +".section .text\n" +"__arch_restore_rt:\n\t" + "movl $0xf, %eax\n\t" // __NR_rt_sigreturn == 0xf + "syscall\n\t" // %rsp must point to a valid struct rt_sigframe. + "int3" +); + +#define __HAVE_ARCH_RESTORE_RT + #endif // _NOLIBC_ARCH_X86_64_H diff --git a/tools/include/nolibc/sys.h b/tools/include/nolibc/sys.h index 3db1dd8c74ee..91532a2fbe2c 100644 --- a/tools/include/nolibc/sys.h +++ b/tools/include/nolibc/sys.h @@ -1026,6 +1026,86 @@ 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)); +} + +static __attribute__((unused)) +int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact) +{ +#ifdef __HAVE_ARCH_RESTORE_RT + 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. + * + * The "signal trampoline" function has to be written in + * inline Assembly to prevent the compiler from controlling + * the %rsp (e.g., with -fno-omit-frame-pointer, every + * function has a `pushq %rbp` that makes the %rsp no longer + * point to `struct rt_sigframe`). + * + * `struct rt_sigframe` contains the registers' value before + * the signal is caught. + * + */ + if (!act2.sa_restorer) { + act2.sa_flags |= SA_RESTORER; + act2.sa_restorer = __arch_restore_rt; + } + + ret = sys_sigaction(signum, &act2, oldact); + if (ret < 0) { + SET_ERRNO(-ret); + ret = -1; + } + return ret; +#else + /* + * TODO: Implement sa_restorer ("signal trampoline") for + * other architectures. + */ + SET_ERRNO(ENOSYS); + return -1; +#endif +} +
/* * int stat(const char *path, struct stat *buf);