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 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@xxxxxxxxxxx>
---
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);
--
Ammar Faizi
