[PATCH v2 1/4] nolibc/sys: Implement `sigaction(2)` function

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

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 __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@1wt.eu
Suggested-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---

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