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/* Create new context.
Copyright (C) 2002-2025 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 <sysdep.h>
#include <stdarg.h>
#include <stdint.h>
#include <ucontext.h>
#if SHSTK_ENABLED
# include <pthread.h>
# include <libc-pointer-arith.h>
# include <sys/prctl.h>
# include <allocate-shadow-stack.h>
#endif
#include "ucontext_i.h"
/* This implementation can handle any ARGC value but only
normal integer parameters.
makecontext sets up a stack and the registers for the
user context. The stack looks like this:
+-----------------------+
| next context |
+-----------------------+
| parameter 7-n |
+-----------------------+
| trampoline address |
%rsp -> +-----------------------+
The registers are set up like this:
%rdi,%rsi,%rdx,%rcx,%r8,%r9: parameter 1 to 6
%rbx : address of next context
%rsp : stack pointer.
*/
/* XXX: This implementation currently only handles integer arguments.
To handle long int and pointer arguments the va_arg arguments needs
to be changed to long and also the stdlib/tst-setcontext.c file needs
to be changed to pass long arguments to makecontext. */
void
__makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
{
extern void __start_context (void) attribute_hidden;
extern void __push___start_context (ucontext_t *)
attribute_hidden;
greg_t *sp;
unsigned int idx_uc_link;
va_list ap;
int i;
/* Generate room on stack for parameter if needed and uc_link. */
sp = (greg_t *) ((uintptr_t) ucp->uc_stack.ss_sp
+ ucp->uc_stack.ss_size);
sp -= (argc > 6 ? argc - 6 : 0) + 1;
/* Align stack and make space for trampoline address. */
sp = (greg_t *) ((((uintptr_t) sp) & -16L) - 8);
idx_uc_link = (argc > 6 ? argc - 6 : 0) + 1;
/* Setup context ucp. */
/* Address to jump to. */
ucp->uc_mcontext.gregs[REG_RIP] = (uintptr_t) func;
/* Setup rbx.*/
ucp->uc_mcontext.gregs[REG_RBX] = (uintptr_t) &sp[idx_uc_link];
ucp->uc_mcontext.gregs[REG_RSP] = (uintptr_t) sp;
/* Setup stack. */
#if SHSTK_ENABLED
struct pthread *self = THREAD_SELF;
unsigned int feature_1 = THREAD_GETMEM (self, header.feature_1);
/* NB: We must check feature_1 before accessing __ssp since caller
may be compiled against ucontext_t without __ssp. */
if ((feature_1 & X86_FEATURE_1_SHSTK) != 0)
{
/* Shadow stack is enabled. We need to allocate a new shadow
stack. NB:
ucp->__ssp[0]: The new shadow stack pointer.
ucp->__ssp[1]: The base address of the new shadow stack.
ucp->__ssp[2]: The size of the new shadow stack.
*/
long int ret
= __allocate_shadow_stack (((uintptr_t) sp
- (uintptr_t) ucp->uc_stack.ss_sp),
&ucp->__ssp[1]);
if (ret != 0)
{
/* FIXME: What should we do? */
abort ();
}
ucp->__ssp[0] = ucp->__ssp[1] + ucp->__ssp[2] - 8;
/* Call __push___start_context to push __start_context onto the new
stack as well as the new shadow stack. */
__push___start_context (ucp);
}
else
#endif
sp[0] = (uintptr_t) &__start_context;
sp[idx_uc_link] = (uintptr_t) ucp->uc_link;
va_start (ap, argc);
/* Handle arguments.
The standard says the parameters must all be int values. This is
an historic accident and would be done differently today. For
x86-64 all integer values are passed as 64-bit values and
therefore extending the API to copy 64-bit values instead of
32-bit ints makes sense. It does not break existing
functionality and it does not violate the standard which says
that passing non-int values means undefined behavior. */
for (i = 0; i < argc; ++i)
switch (i)
{
case 0:
ucp->uc_mcontext.gregs[REG_RDI] = va_arg (ap, greg_t);
break;
case 1:
ucp->uc_mcontext.gregs[REG_RSI] = va_arg (ap, greg_t);
break;
case 2:
ucp->uc_mcontext.gregs[REG_RDX] = va_arg (ap, greg_t);
break;
case 3:
ucp->uc_mcontext.gregs[REG_RCX] = va_arg (ap, greg_t);
break;
case 4:
ucp->uc_mcontext.gregs[REG_R8] = va_arg (ap, greg_t);
break;
case 5:
ucp->uc_mcontext.gregs[REG_R9] = va_arg (ap, greg_t);
break;
default:
/* Put value on stack. */
sp[i - 5] = va_arg (ap, greg_t);
break;
}
va_end (ap);
}
weak_alias (__makecontext, makecontext)
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