From: Ammar Faizi <[email protected]>
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/[email protected]
Suggested-by: Willy Tarreau <[email protected]>
Signed-off-by: Ammar Faizi <[email protected]>
---
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