sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305]

Add _SC_MINSIGSTKSZ for the minimum signal stack size derived from
AT_MINSIGSTKSZ, which is the minimum number of bytes of free stack
space required in order to gurantee successful, non-nested handling
of a single signal whose handler is an empty function, and _SC_SIGSTKSZ
which is the suggested minimum number of bytes of stack space required
for a signal stack.

If AT_MINSIGSTKSZ isn't available, sysconf (_SC_MINSIGSTKSZ) returns
MINSIGSTKSZ.  On Linux/x86 with XSAVE, the signal frame used by kernel
is composed of the following areas and laid out as:

 ------------------------------
 | alignment padding          |
 ------------------------------
 | xsave buffer               |
 ------------------------------
 | fsave header (32-bit only) |
 ------------------------------
 | siginfo + ucontext         |
 ------------------------------

Compute AT_MINSIGSTKSZ value as size of xsave buffer + size of fsave
header (32-bit only) + size of siginfo and ucontext + alignment padding.

If _SC_SIGSTKSZ_SOURCE or _GNU_SOURCE are defined, MINSIGSTKSZ and SIGSTKSZ
are redefined as

/* Default stack size for a signal handler: sysconf (SC_SIGSTKSZ).  */
 # undef SIGSTKSZ
 # define SIGSTKSZ sysconf (_SC_SIGSTKSZ)

/* Minimum stack size for a signal handler: SIGSTKSZ.  */
 # undef MINSIGSTKSZ
 # define MINSIGSTKSZ SIGSTKSZ

Compilation will fail if the source assumes constant MINSIGSTKSZ or
SIGSTKSZ.

The reason for not simply increasing the kernel's MINSIGSTKSZ #define
(apart from the fact that it is rarely used, due to glibc's shadowing
definitions) was that userspace binaries will have baked in the old
value of the constant and may be making assumptions about it.

For example, the type (char [MINSIGSTKSZ]) changes if this #define
changes.  This could be a problem if an newly built library tries to
memcpy() or dump such an object defined by and old binary.
Bounds-checking and the stack sizes passed to things like sigaltstack()
and makecontext() could similarly go wrong.

3 files changed, 88 insertions, 2 deletions

diff --git a/signal/Makefile b/signal/Makefile
index e89c308..3d8cab1 100644
--- a/signal/Makefile
+++ b/signal/Makefile
@@ -31,7 +31,8 @@ headers := signal.h sys/signal.h \
 	   bits/types/sigevent_t.h bits/types/siginfo_t.h \
 	   bits/types/sigset_t.h bits/types/sigval_t.h \
 	   bits/types/stack_t.h bits/types/struct_sigstack.h \
-	   bits/types/__sigval_t.h bits/signal_ext.h
+	   bits/types/__sigval_t.h bits/signal_ext.h \
+	   bits/sigstksz.h
 
 routines	:= signal raise killpg \
 		   sigaction sigprocmask kill \
@@ -48,7 +49,7 @@ routines	:= signal raise killpg \
 tests		:= tst-signal tst-sigset tst-sigsimple tst-raise tst-sigset2 \
 		   tst-sigwait-eintr tst-sigaction \
 		   tst-minsigstksz-1 tst-minsigstksz-2 tst-minsigstksz-3 \
-		   tst-minsigstksz-3a tst-minsigstksz-4 \
+		   tst-minsigstksz-3a tst-minsigstksz-4 tst-minsigstksz-5 \
 		   tst-sigisemptyset
 
 include ../Rules
diff --git a/signal/signal.h b/signal/signal.h
index 6463c98..b17203c 100644
--- a/signal/signal.h
+++ b/signal/signal.h
@@ -312,6 +312,7 @@ extern int siginterrupt (int __sig, int __interrupt) __THROW
   __attribute_deprecated_msg__ ("Use sigaction with SA_RESTART instead");
 
 # include <bits/sigstack.h>
+# include <bits/sigstksz.h>
 # include <bits/ss_flags.h>
 
 /* Alternate signal handler stack interface.
diff --git a/signal/tst-minsigstksz-5.c b/signal/tst-minsigstksz-5.c
new file mode 100644
index 0000000..d657d2f
--- /dev/null
+++ b/signal/tst-minsigstksz-5.c
@@ -0,0 +1,84 @@
+/* Test of signal delivery on an alternate stack with MINSIGSTKSZ size.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <signal.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <support/check.h>
+#include <support/support.h>
+
+static volatile sig_atomic_t handler_run;
+
+static void
+handler (int signo)
+{
+  /* Clear a bit of on-stack memory.  */
+  volatile char buffer[256];
+  for (size_t i = 0; i < sizeof (buffer); ++i)
+    buffer[i] = 0;
+  handler_run = 1;
+}
+
+int
+do_test (void)
+{
+  size_t stack_buffer_size = 64 * 1024 * 1024;
+  void *stack_buffer = xmalloc (stack_buffer_size);
+  void *stack_end = stack_buffer + stack_buffer_size;
+  memset (stack_buffer, 0xCC, stack_buffer_size);
+
+  void *stack_bottom = stack_buffer + (stack_buffer_size + MINSIGSTKSZ) / 2;
+  void *stack_top = stack_bottom + MINSIGSTKSZ;
+  stack_t stack =
+    {
+      .ss_sp = stack_bottom,
+      .ss_size = MINSIGSTKSZ,
+    };
+  if (sigaltstack (&stack, NULL) < 0)
+    FAIL_RET ("sigaltstack: %m\n");
+
+  struct sigaction act =
+    {
+      .sa_handler = handler,
+      .sa_flags = SA_ONSTACK,
+    };
+  if (sigaction (SIGUSR1, &act, NULL) < 0)
+    FAIL_RET ("sigaction: %m\n");
+
+  if (kill (getpid (), SIGUSR1) < 0)
+    FAIL_RET ("kill: %m\n");
+
+  if (handler_run != 1)
+    FAIL_RET ("handler did not run\n");
+
+  for (void *p = stack_buffer; p < stack_bottom; ++p)
+    if (*(unsigned char *) p != 0xCC)
+      FAIL_RET ("changed byte %ld bytes below configured stack\n",
+		(long) (stack_bottom - p));
+  for (void *p = stack_top; p < stack_end; ++p)
+    if (*(unsigned char *) p != 0xCC)
+      FAIL_RET ("changed byte %ld bytes above configured stack\n",
+		(long) (p - stack_top));
+
+  free (stack_buffer);
+
+  return 0;
+}
+
+#include <support/test-driver.c>