Re: [PATCH v2 4/9] bsd-user/syscall: Replace alloca() by g_new()

[Peter Maydell]

On Thu, 6 May 2021 at 15:17, Warner Losh <imp@bsdimp.com> wrote:
>
>
>
> On Thu, May 6, 2021, 7:38 AM Philippe Mathieu-Daudé <philmd@redhat.com> wrote:
>>
>> The ALLOCA(3) man-page mentions its "use is discouraged".
>>
>> Replace it by a g_new() call.
>>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>>  bsd-user/syscall.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/bsd-user/syscall.c b/bsd-user/syscall.c
>> index 4abff796c76..dbee0385ceb 100644
>> --- a/bsd-user/syscall.c
>> +++ b/bsd-user/syscall.c
>> @@ -355,9 +355,8 @@ abi_long do_freebsd_syscall(void *cpu_env, int num, 
>> abi_long arg1,
>>      case TARGET_FREEBSD_NR_writev:
>>          {
>>              int count = arg3;
>> -            struct iovec *vec;
>> +            g_autofree struct iovec *vec = g_new(struct iovec, count);
>
>
> Where is this freed?

g_autofree, so it gets freed when it goes out of scope.
https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html#g-autofree

> Also, alloca just moves a stack pointer, where malloc has complex 
> interactions. Are you sure that's a safe change here?

alloca()ing something with size determined by the guest is
definitely not safe :-) malloc as part of "handle this syscall"
is pretty common, at least in linux-user.

thanks
-- PMM