Cygwin: signal: Copy context to alternate stack in the SA_ONSTACK case

After the commit 0210c77, the context passed to signal handler
cannot be accessed from the signal handler that uses alternate stack.
This is because the context locally copied is on the stack that is
different area from the signal handler uses. With this patch, copy
the context to alternate signal stack area to avoid this situation.

Backported-from: 7f67575 (Cygwin: signal: Copy context to alternate stack in the SA_ONSTACK case, 2025-03-25)
Addresses: https://cygwin.com/pipermail/cygwin/2025-March/257714.html
Fixes: 0210c77 ("Cygwin: signal: Use context locally copied in call_signal_handler()")
Reported-by: Bruno Haible <[email protected]>
Reviewed-by: Corinna Vischen <[email protected]>
Signed-off-by: Takashi Yano <[email protected]>
(cherry picked from commit 0d0e76b)
Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

winsup/cygwin/exceptions.cc

@@ -1807,6 +1807,13 @@ _cygtls::call_signal_handler ()
 	     to 16 byte. */
 	  uintptr_t new_sp = ((uintptr_t) _my_tls.altstack.ss_sp
 			      + _my_tls.altstack.ss_size) & ~0xf;
+	  /* Copy context1 to the alternate signal stack area, because the
+	     context1 allocated in the normal stack area is not accessible
+	     from the signal handler that uses alternate signal stack. */
+	  thiscontext = (ucontext_t *) ((new_sp - sizeof (ucontext_t)) & ~0xf);
+	  memcpy (thiscontext, &context1, sizeof (ucontext_t));
+	  new_sp = (uintptr_t) thiscontext;
+
 	  /* In assembler: Save regs on new stack, move to alternate stack,
 	     call thisfunc, revert stack regs. */
 #ifdef __x86_64__
@@ -1850,6 +1857,7 @@ _cygtls::call_signal_handler ()
 #else
 #error unimplemented for this target
 #endif
+	  memcpy (&context1, thiscontext, sizeof (ucontext_t));
 	}
       else
 	/* No alternate signal stack requested or available, just call

winsup/cygwin/release/3.6.1

@@ -6,3 +6,8 @@ Fixes:
 
 - Clear direction flag in sigdeleyed before calling signal handler.
   Addresses: https://cygwin.com/pipermail/cygwin/2025-March/257704.html
+
+- Copy context to alternate signal stack area in call_signal_handler()
+  in the SA_ONSTACK case, because locally-copied context on the normal
+  stack area is not accessible from the signal handler.
+  Addresses: https://cygwin.com/pipermail/cygwin/2025-March/257714.html