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/* sigaltstack wrappers.
Copyright (C) 2019-2020 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 <support/xsignal.h>
#include <support/support.h>
#include <support/xunistd.h>
#include <support/check.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/param.h> /* roundup, MAX */
#ifndef MAP_NORESERVE
# define MAP_NORESERVE 0
#endif
#ifndef MAP_STACK
# define MAP_STACK 0
#endif
/* The "cookie" returned by xalloc_sigstack points to one of these
structures. */
struct sigstack_desc
{
void *alloc_base; /* Base address of the complete allocation. */
size_t alloc_size; /* Size of the complete allocation. */
stack_t alt_stack; /* The address and size of the stack itself. */
stack_t old_stack; /* The previous signal stack. */
};
void *
xalloc_sigstack (size_t size)
{
size_t pagesize = sysconf (_SC_PAGESIZE);
if (pagesize == -1)
FAIL_EXIT1 ("sysconf (_SC_PAGESIZE): %m\n");
/* Always supply at least MINSIGSTKSZ space; passing 0 as size means
only that much space. No matter what the number is, round it up
to a whole number of pages. */
size_t stacksize = roundup (size + MINSIGSTKSZ, pagesize);
/* The guard bands need to be large enough to intercept offset
accesses from a stack address that might otherwise hit another
mapping. Make them at least twice as big as the stack itself, to
defend against an offset by the entire size of a large
stack-allocated array. The minimum is 1MiB, which is arbitrarily
chosen to be larger than any "typical" wild pointer offset.
Again, no matter what the number is, round it up to a whole
number of pages. */
size_t guardsize = roundup (MAX (2 * stacksize, 1024 * 1024), pagesize);
struct sigstack_desc *desc = xmalloc (sizeof (struct sigstack_desc));
desc->alloc_size = guardsize + stacksize + guardsize;
/* Use MAP_NORESERVE so that RAM will not be wasted on the guard
bands; touch all the pages of the actual stack before returning,
so we know they are allocated. */
desc->alloc_base = xmmap (0,
desc->alloc_size,
PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE|MAP_STACK,
-1);
xmprotect (desc->alloc_base, guardsize, PROT_NONE);
xmprotect (desc->alloc_base + guardsize + stacksize, guardsize, PROT_NONE);
memset (desc->alloc_base + guardsize, 0xA5, stacksize);
desc->alt_stack.ss_sp = desc->alloc_base + guardsize;
desc->alt_stack.ss_flags = 0;
desc->alt_stack.ss_size = stacksize;
if (sigaltstack (&desc->alt_stack, &desc->old_stack))
FAIL_EXIT1 ("sigaltstack (new stack: sp=%p, size=%zu, flags=%u): %m\n",
desc->alt_stack.ss_sp, desc->alt_stack.ss_size,
desc->alt_stack.ss_flags);
return desc;
}
void
xfree_sigstack (void *stack)
{
struct sigstack_desc *desc = stack;
if (sigaltstack (&desc->old_stack, 0))
FAIL_EXIT1 ("sigaltstack (restore old stack: sp=%p, size=%zu, flags=%u): "
"%m\n", desc->old_stack.ss_sp, desc->old_stack.ss_size,
desc->old_stack.ss_flags);
xmunmap (desc->alloc_base, desc->alloc_size);
free (desc);
}
void
xget_sigstack_location (const void *stack, unsigned char **addrp, size_t *sizep)
{
const struct sigstack_desc *desc = stack;
*addrp = desc->alt_stack.ss_sp;
*sizep = desc->alt_stack.ss_size;
}
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