Re: [PATCH] linux: Change syscall return value to long int

[Szabolcs Nagy via Libc-alpha <libc-alpha@sourceware.org>]

The 11/29/2022 11:16, XingLi wrote:
> From: Xing Li <lixing@loongson.cn>
> 
> The kernel syscall return is long value.
> The generic syscall interface return value
> is int, which may lead to incorrect return value.

it's not clear what you mean here, the generic syscall
function returns long (according to unistd.h).

> 
> The following test is syscall with mmap executed on LoongArch,
> only 32bits and sign extension value returned leading to mmap failure,
> which should be with 47bits address returned.
> 
> Testcase:
> 
>  #include <sys/syscall.h>
>  #include <sys/mman.h>
>  #include <stdio.h>
> 
> void main()
> {
>         long int ret;
>         ret = syscall(SYS_mmap, NULL, 0x801000, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, -1, 0);

Note: there are many reasons why direct calls to syscall
may not work.

syscall is a variadic argument function that takes long
arguments, but you pass ints that may *not* be sign/zero
extended on the caller site so e.g. the top 32bits of
size and offset can be arbitrary on a 64bit system.
(you have to cast args to long to make the example valid).

and some systems use SYS_mmap2.

>         printf("map address is %lx\n",ret);
> }
> 
> Result:
> [lixing@Sunhaiyong test]$ ./mmap
> map address is fffffffff008c000
> ---
>  sysdeps/unix/sysv/linux/syscall.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/sysdeps/unix/sysv/linux/syscall.c b/sysdeps/unix/sysv/linux/syscall.c
> index 7303ba7188..8cb0b66b1c 100644
> --- a/sysdeps/unix/sysv/linux/syscall.c
> +++ b/sysdeps/unix/sysv/linux/syscall.c
> @@ -33,7 +33,7 @@ syscall (long int number, ...)
>    long int a5 = va_arg (args, long int);
>    va_end (args);
>  
> -  int r = INTERNAL_SYSCALL_NCS_CALL (number, a0, a1, a2, a3, a4, a5);
> +  long int r = INTERNAL_SYSCALL_NCS_CALL (number, a0, a1, a2, a3, a4, a5);
>    if (__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (r)))

this change looks reasonable to me.

>      {
>        __set_errno (-r);
> -- 
> 2.31.1
>