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@@ -0,0 +1,106 @@
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+c-stack: stop using SIGSTKSZ
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+
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+It’s been proposed to stop making SIGSTKSZ an integer constant:
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+https://sourceware.org/pipermail/libc-alpha/2020-September/118028.html
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+Also, using SIGSTKSZ in #if did not conform to current POSIX.
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+Also, avoiding SIGSTKSZ makes the code simpler and easier to grok.
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+* lib/c-stack.c (SIGSTKSZ): Remove.
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+(alternate_signal_stack): Now a 64 KiB array, for simplicity.
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+All uses changed.
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+
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+[Retrieved (and backported) from:
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+https://git.savannah.gnu.org/cgit/gnulib.git/patch/?id=f9e2b20a12a230efa30f1d479563ae07d276a94b]
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+Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
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+
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+diff -Nura m4-1.4.18.orig/lib/c-stack.c m4-1.4.18/lib/c-stack.c
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+--- m4-1.4.18.orig/lib/c-stack.c 2021-04-11 19:12:14.086494029 +0200
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++++ m4-1.4.18/lib/c-stack.c 2021-04-11 19:48:46.316862760 +0200
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+@@ -50,15 +50,16 @@
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+ #if ! HAVE_STACK_T && ! defined stack_t
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+ typedef struct sigaltstack stack_t;
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+ #endif
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+-#ifndef SIGSTKSZ
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+-# define SIGSTKSZ 16384
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+-#elif HAVE_LIBSIGSEGV && SIGSTKSZ < 16384
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+-/* libsigsegv 2.6 through 2.8 have a bug where some architectures use
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+- more than the Linux default of an 8k alternate stack when deciding
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+- if a fault was caused by stack overflow. */
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+-# undef SIGSTKSZ
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+-# define SIGSTKSZ 16384
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+-#endif
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++
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++/* Storage for the alternate signal stack.
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++ 64 KiB is not too large for Gnulib-using apps, and is large enough
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++ for all known platforms. Smaller sizes may run into trouble.
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++ For example, libsigsegv 2.6 through 2.8 have a bug where some
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++ architectures use more than the Linux default of an 8 KiB alternate
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++ stack when deciding if a fault was caused by stack overflow. */
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++static max_align_t alternate_signal_stack[(64 * 1024
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++ + sizeof (max_align_t) - 1)
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++ / sizeof (max_align_t)];
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+
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+ #include <stdlib.h>
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+ #include <string.h>
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+@@ -128,19 +129,6 @@
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+ #if (HAVE_SIGALTSTACK && HAVE_DECL_SIGALTSTACK \
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+ && HAVE_STACK_OVERFLOW_HANDLING) || HAVE_LIBSIGSEGV
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+
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+-/* Storage for the alternate signal stack. */
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+-static union
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+-{
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+- char buffer[SIGSTKSZ];
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+-
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+- /* These other members are for proper alignment. There's no
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+- standard way to guarantee stack alignment, but this seems enough
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+- in practice. */
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+- long double ld;
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+- long l;
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+- void *p;
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+-} alternate_signal_stack;
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+-
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+ static void
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+ null_action (int signo __attribute__ ((unused)))
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+ {
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+@@ -205,8 +193,8 @@
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+
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+ /* Always install the overflow handler. */
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+ if (stackoverflow_install_handler (overflow_handler,
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+- alternate_signal_stack.buffer,
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+- sizeof alternate_signal_stack.buffer))
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++ alternate_signal_stack,
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++ sizeof alternate_signal_stack))
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+ {
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+ errno = ENOTSUP;
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+ return -1;
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+@@ -279,14 +267,14 @@
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+ stack_t st;
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+ struct sigaction act;
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+ st.ss_flags = 0;
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++ st.ss_sp = alternate_signal_stack;
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++ st.ss_size = sizeof alternate_signal_stack;
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+ # if SIGALTSTACK_SS_REVERSED
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+ /* Irix mistakenly treats ss_sp as the upper bound, rather than
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+ lower bound, of the alternate stack. */
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+- st.ss_sp = alternate_signal_stack.buffer + SIGSTKSZ - sizeof (void *);
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+- st.ss_size = sizeof alternate_signal_stack.buffer - sizeof (void *);
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+-# else
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+- st.ss_sp = alternate_signal_stack.buffer;
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+- st.ss_size = sizeof alternate_signal_stack.buffer;
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++ st.ss_size -= sizeof (void *);
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++ char *ss_sp = st.ss_sp;
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++ st.ss_sp = ss_sp + st.ss_size;
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+ # endif
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+ r = sigaltstack (&st, NULL);
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+ if (r != 0)
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+diff -Nura m4-1.4.18.orig/lib/c-stack.h m4-1.4.18/lib/c-stack.h
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+--- m4-1.4.18.orig/lib/c-stack.h 2021-04-11 19:12:14.098494042 +0200
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++++ m4-1.4.18/lib/c-stack.h 2021-04-11 19:17:42.138848378 +0200
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+@@ -34,7 +34,7 @@
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+ A null ACTION acts like an action that does nothing.
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+
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+ ACTION must be async-signal-safe. ACTION together with its callees
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+- must not require more than SIGSTKSZ bytes of stack space. Also,
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++ must not require more than 64 KiB bytes of stack space. Also,
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+ ACTION should not call longjmp, because this implementation does
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+ not guarantee that it is safe to return to the original stack.
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+
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Fabrice Fontaine