From f9e2b20a12a230efa30f1d479563ae07d276a94b Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Wed, 30 Sep 2020 13:50:36 -0700
Subject: [PATCH] c-stack: stop using SIGSTKSZ
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It’s been proposed to stop making SIGSTKSZ an integer constant:
https://sourceware.org/pipermail/libc-alpha/2020-September/118028.html
Also, using SIGSTKSZ in #if did not conform to current POSIX.
Also, avoiding SIGSTKSZ makes the code simpler and easier to grok.
* lib/c-stack.c (SIGSTKSZ): Remove.
(alternate_signal_stack): Now a 64 KiB array, for simplicity.
All uses changed.
---
 lib/c-stack.c | 46 +++++++++++++++++-----------------------------
 lib/c-stack.h |  2 +-
 2 files changed, 18 insertions(+), 30 deletions(-)

diff --git a/lib/c-stack.c b/lib/c-stack.c
index 5353c08..7871e9c 100644
--- a/lib/c-stack.c
+++ b/lib/c-stack.c
@@ -50,15 +50,6 @@
 #if ! HAVE_STACK_T && ! defined stack_t
 typedef struct sigaltstack stack_t;
 #endif
-#ifndef SIGSTKSZ
-# define SIGSTKSZ 16384
-#elif HAVE_LIBSIGSEGV && SIGSTKSZ < 16384
-/* libsigsegv 2.6 through 2.8 have a bug where some architectures use
-   more than the Linux default of an 8k alternate stack when deciding
-   if a fault was caused by stack overflow.  */
-# undef SIGSTKSZ
-# define SIGSTKSZ 16384
-#endif
 
 #include <stdlib.h>
 #include <string.h>
@@ -89,6 +80,16 @@ typedef struct sigaltstack stack_t;
 # endif
 #endif
 
+/* Storage for the alternate signal stack.
+   64 KiB is not too large for Gnulib-using apps, and is large enough
+   for all known platforms.  Smaller sizes may run into trouble.
+   For example, libsigsegv 2.6 through 2.8 have a bug where some
+   architectures use more than the Linux default of an 8 KiB alternate
+   stack when deciding if a fault was caused by stack overflow.  */
+static max_align_t alternate_signal_stack[(64 * 1024
+                                           + sizeof (max_align_t) - 1)
+                                          / sizeof (max_align_t)];
+
 /* The user-specified action to take when a SEGV-related program error
    or stack overflow occurs.  */
 static void (* volatile segv_action) (int);
@@ -128,19 +129,6 @@ die (int signo)
 #if (HAVE_SIGALTSTACK && HAVE_DECL_SIGALTSTACK \
      && HAVE_STACK_OVERFLOW_HANDLING) || HAVE_LIBSIGSEGV
 
-/* Storage for the alternate signal stack.  */
-static union
-{
-  char buffer[SIGSTKSZ];
-
-  /* These other members are for proper alignment.  There's no
-     standard way to guarantee stack alignment, but this seems enough
-     in practice.  */
-  long double ld;
-  long l;
-  void *p;
-} alternate_signal_stack;
-
 static void
 null_action (int signo __attribute__ ((unused)))
 {
@@ -205,8 +193,8 @@ c_stack_action (void (*action) (int))
 
   /* Always install the overflow handler.  */
   if (stackoverflow_install_handler (overflow_handler,
-                                     alternate_signal_stack.buffer,
-                                     sizeof alternate_signal_stack.buffer))
+                                     alternate_signal_stack,
+                                     sizeof alternate_signal_stack))
     {
       errno = ENOTSUP;
       return -1;
@@ -279,14 +267,14 @@ c_stack_action (void (*action) (int))
   stack_t st;
   struct sigaction act;
   st.ss_flags = 0;
+  st.ss_sp = alternate_signal_stack;
+  st.ss_size = sizeof alternate_signal_stack;
 # if SIGALTSTACK_SS_REVERSED
   /* Irix mistakenly treats ss_sp as the upper bound, rather than
      lower bound, of the alternate stack.  */
-  st.ss_sp = alternate_signal_stack.buffer + SIGSTKSZ - sizeof (void *);
-  st.ss_size = sizeof alternate_signal_stack.buffer - sizeof (void *);
-# else
-  st.ss_sp = alternate_signal_stack.buffer;
-  st.ss_size = sizeof alternate_signal_stack.buffer;
+  st.ss_size -= sizeof (void *);
+  char *ss_sp = st.ss_sp;
+  st.ss_sp = ss_sp + st.ss_size;
 # endif
   r = sigaltstack (&st, NULL);
   if (r != 0)
diff --git a/lib/c-stack.h b/lib/c-stack.h
index efd3b8f..df8a790 100644
--- a/lib/c-stack.h
+++ b/lib/c-stack.h
@@ -34,7 +34,7 @@
    A null ACTION acts like an action that does nothing.
 
    ACTION must be async-signal-safe.  ACTION together with its callees
-   must not require more than SIGSTKSZ bytes of stack space.  Also,
+   must not require more than 64 KiB of stack space.  Also,
    ACTION should not call longjmp, because this implementation does
    not guarantee that it is safe to return to the original stack.
 
-- 
1.8.3.1

