[PATCH v5 14/16] riscv/cfi: Add __allocate_shadow_stack for mapping new shadow stack

[Gabriel Ravier]

On 9/13/26 9:38 PM, Gabriel Ravier wrote:
> On 6/28/26 9:02 AM, Jesse Huang wrote:
>> Co-authored-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
>> Co-authored-by: Jerry Zhang Jian <jerry.zhangjian@sifive.com>
>> ---
>>   sysdeps/unix/sysv/linux/riscv/Makefile        |  1 +
>>   .../sysv/linux/riscv/allocate-shadow-stack.c  | 59 +++++++++++++++++++
>>   .../sysv/linux/riscv/allocate-shadow-stack.h  | 31 ++++++++++
>>   sysdeps/unix/sysv/linux/riscv/bits/mman.h     | 30 ++++++++++
>>   sysdeps/unix/sysv/linux/riscv/sysdep.h        |  2 +
>>   5 files changed, 123 insertions(+)
>>   create mode 100644 
>> sysdeps/unix/sysv/linux/riscv/allocate-shadow-stack.c
>>   create mode 100644 
>> sysdeps/unix/sysv/linux/riscv/allocate-shadow-stack.h
>>   create mode 100644 sysdeps/unix/sysv/linux/riscv/bits/mman.h
>>
>> [snip]
>> diff --git a/sysdeps/unix/sysv/linux/riscv/allocate-shadow-stack.c 
>> b/sysdeps/unix/sysv/linux/riscv/allocate-shadow-stack.c
>> new file mode 100644
>> index 0000000000..e64ddb2c56
>> --- /dev/null
>> +++ b/sysdeps/unix/sysv/linux/riscv/allocate-shadow-stack.c
>> @@ -0,0 +1,59 @@
>> +/* Helper function to allocate shadow stack.
>> +   Copyright (C) 2023-2026 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 <stdint.h>
>> +#include <errno.h>
>> +#include <sys/mman.h>
>> +#include <libc-pointer-arith.h>
>> +#include <allocate-shadow-stack.h>
>> +
>> +#ifndef SHADOW_STACK_SET_TOKEN
>> +# define SHADOW_STACK_SET_TOKEN 0
>> +#endif
>> +
>> +/* NB: This can be treated as a syscall by caller.  */
>> +
>> +long int
>> +__allocate_shadow_stack (size_t stack_size,
>> +             shadow_stack_size_t *child_stack)
>> +{
>> +#ifdef __NR_map_shadow_stack
>> +  size_t shadow_stack_size
>> +    = stack_size >> STACK_SIZE_TO_SHADOW_STACK_SIZE_SHIFT;
>> +  /* Align shadow stack to 8 bytes.  */
>> +  shadow_stack_size = ALIGN_UP (shadow_stack_size, 8);
>> +  /* Since sigaltstack shares shadow stack with the current context in
>> +     the thread, add extra 20 stack frames in shadow stack for signal
>> +     handlers.  */
>> +  shadow_stack_size += 20 * 8;
>> +  void *shadow_stack = (void *)INLINE_SYSCALL_CALL
>> +    (map_shadow_stack, NULL, shadow_stack_size, 
>> SHADOW_STACK_SET_TOKEN);
>> +  /* Report the map_shadow_stack error.  */
>> +  if (shadow_stack < 0)
>> +    return -errno;
>
>
> This comparison seems rather strange to me. Relational comparison with 
> a void * generally seems like something GCC would warn on. I'm also 
> wondering where INLINE_SYSCALL_CALL is from ? I don't actually see it 
> anywhere in the glibc source code or in the rest of the patch...


nvm on the INLINE_SYSCALL_CALL, idk how I didn't find it it's right 
there x) (the rest of this remark is still applicable, AIUI).

>
>> +
>> +  /* Save the shadow stack base and size on child stack.  */
>> +  child_stack[0] = (uintptr_t) shadow_stack;
>> +  child_stack[1] = shadow_stack_size;
>> +
>> +  return 0;
>> +#else
>> +  return -ENOSYS;
>> +#endif
>> +}
>> [snip]
>
>