[gdb/cli] Warn about forced return from signal trampoline

The Linaro CI reported a regression on arm-linux in test-case
gdb.base/sigstep.exp following commit 7b46460a619 ("[gdb/symtab] Apply
workaround for PR gas/31115 a bit more") [1]:
...
(gdb) return^M
Make __default_sa_restorer return now? (y or n) n^M
Not confirmed^M
(gdb) FAIL: $exp: return from handleri: \
  leave signal trampoline (got interactive prompt)
...

After installing package glibc-debuginfo and adding --with-separate-debug-dir
to the configure flags, I managed to reproduce the FAIL.

The regression seems to be a progression in the sense that the function name
for the signal trampoline is found.

After reading up on the signal trampoline [2] and the return command [3], my
understanding is that forced returning from the signal trampoline is
potentially unsafe, given that for instance the process signal mask won't be
restored.

Fix this by:
- rather than using the name, using "signal trampoline" in the query, and
- adding a warning about returning from a signal trampoline,
giving us:
...
(gdb) return^M
warning: Returning from signal trampoline does not fully restore pre-signal \
  state, such as process signal mask.^M
Make signal trampoline return now? (y or n) y^M
87            dummy = 0; dummy = 0; while (!done);^M
(gdb) PASS: $exp: return from handleri: leave signal trampoline (in main)
...

Tested on x86_64-linux.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>

[1] https://linaro.atlassian.net/browse/GNU-1459
[2] https://man7.org/linux/man-pages/man2/sigreturn.2.html
[3] https://sourceware.org/gdb/current/onlinedocs/gdb.html/Returning.html

2 files changed, 9 insertions, 2 deletions

diff --git a/gdb/stack.c b/gdb/stack.c
index 4a3e7e4..9785a94 100644
--- a/gdb/stack.c
+++ b/gdb/stack.c
@@ -2776,7 +2776,14 @@ return_command (const char *retval_exp, int from_tty)
     {
       int confirmed;
 
-      if (thisfun == NULL)
+      if (get_frame_type (thisframe) == SIGTRAMP_FRAME)
+	{
+	  warning (_("Returning from signal trampoline does not fully restore"
+		     " pre-signal state, such as process signal mask."));
+	  confirmed = query (_("%sMake signal trampoline return now? "),
+			     query_prefix.c_str ());
+	}
+      else if (thisfun == NULL)
 	confirmed = query (_("%sMake selected stack frame return now? "),
 			   query_prefix.c_str ());
       else
diff --git a/gdb/testsuite/gdb.base/sigstep.exp b/gdb/testsuite/gdb.base/sigstep.exp
index 315d89d..d08a34e 100644
--- a/gdb/testsuite/gdb.base/sigstep.exp
+++ b/gdb/testsuite/gdb.base/sigstep.exp
@@ -259,7 +259,7 @@ proc advancei { cmd } {
 	    -re "return .*${gdb_prompt} $" {
 		fail "$test (stepped)"
 	    }
-	    -re "Make .*frame return now.*y or n. $" {
+	    -re "Make signal trampoline return now.*y or n. $" {
 		send_gdb "y\n"
 		exp_continue -continue_timer
 	    }