Re: [PATCH v16 16/16] selftests, arm64: add a selftest for passing tagged pointers to kernel

[Andrey Konovalov]

On Tue, Jun 11, 2019 at 5:01 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>
> On Mon, Jun 03, 2019 at 06:55:18PM +0200, Andrey Konovalov wrote:
> > This patch is a part of a series that extends arm64 kernel ABI to allow to
> > pass tagged user pointers (with the top byte set to something else other
> > than 0x00) as syscall arguments.
> >
> > This patch adds a simple test, that calls the uname syscall with a
> > tagged user pointer as an argument. Without the kernel accepting tagged
> > user pointers the test fails with EFAULT.
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
>
> BTW, you could add
>
> Co-developed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
>
> since I wrote the malloc() etc. hooks.

Sure!

>
>
> > +static void *tag_ptr(void *ptr)
> > +{
> > +     unsigned long tag = rand() & 0xff;
> > +     if (!ptr)
> > +             return ptr;
> > +     return (void *)((unsigned long)ptr | (tag << TAG_SHIFT));
> > +}
>
> With the prctl() option, this function becomes (if you have a better
> idea, fine by me):
>
> ----------8<---------------
> #include <stdlib.h>
> #include <sys/prctl.h>
>
> #define TAG_SHIFT       (56)
> #define TAG_MASK        (0xffUL << TAG_SHIFT)
>
> #define PR_SET_TAGGED_ADDR_CTRL         55
> #define PR_GET_TAGGED_ADDR_CTRL         56
> # define PR_TAGGED_ADDR_ENABLE          (1UL << 0)
>
> void *__libc_malloc(size_t size);
> void __libc_free(void *ptr);
> void *__libc_realloc(void *ptr, size_t size);
> void *__libc_calloc(size_t nmemb, size_t size);
>
> static void *tag_ptr(void *ptr)
> {
>         static int tagged_addr_err = 1;
>         unsigned long tag = 0;
>
>         if (tagged_addr_err == 1)
>                 tagged_addr_err = prctl(PR_SET_TAGGED_ADDR_CTRL,
>                                         PR_TAGGED_ADDR_ENABLE, 0, 0, 0);

I think this requires atomics. malloc() can be called from multiple threads.

>
>         if (!ptr)
>                 return ptr;
>         if (!tagged_addr_err)
>                 tag = rand() & 0xff;
>
>         return (void *)((unsigned long)ptr | (tag << TAG_SHIFT));
> }
>
> --
> Catalin