Re: [PATCH] selftests: mm: Fix build errors on armhf

[Jeff Xu <jeffxu@chromium.org>]

Hi Muhammad

On Mon, Aug 19, 2024 at 3:05 AM Muhammad Usama Anjum
<Usama.Anjum@collabora.com> wrote:
>
> On 8/14/24 3:29 AM, Jeff Xu wrote:
> > Hi Muhammad
> >
> > On Fri, Aug 9, 2024 at 1:25 AM Muhammad Usama Anjum
> > <usama.anjum@collabora.com> wrote:
> >>
> >> The __NR_mmap isn't found on armhf. The mmap() is commonly available
> >> system call and its wrapper is presnet on all architectures. So it
> >> should be used directly. It solves problem for armhf and doesn't create
> >> problem for architectures as well. Remove sys_mmap() functions as they
> >> aren't doing anything else other than calling mmap(). There is no need
> >> to set errno = 0 manually as glibc always resets it.
> >>
> > The mseal_test should't have dependency on libc, and mmap() is
> > implemented by glibc, right ?
> >
> > I just fixed a bug to switch mremap() to sys_mremap to address an
> > issue that different glibc version's behavior is slightly different
> > for mremap().
> >
> > What is the reason that __NR_mmap not available in armhf ? (maybe it
> > is another name ?)  there must be a way to call syscall directly on
> > armhf, can we use that instead ?
>
> It seems __NR_mmap syscall is deprecated for arm. Found this comment in
> arch/arm/include/asm/unistd.h:
> /*
>  * The following syscalls are obsolete and no longer available for EABI:
>  *  __NR_time
>  *  __NR_umount
>  *  __NR_stime
>  *  __NR_alarm
>  *  __NR_utime
>  *  __NR_getrlimit
>  *  __NR_select
>  *  __NR_readdir
>  *  __NR_mmap
>  *  __NR_socketcall
>  *  __NR_syscall
>  *  __NR_ipc
>  */
>
> The glibc mmap() calls mmap2() these days by adjusting the parameters
> internally. From man mmap:
> C library/kernel differences:
> This  page  describes the interface provided by the glibc mmap() wrapper
> function.  Originally, this function invoked a system call of the same
> name.  Since Linux 2.4, that system call has been superseded  by
> mmap2(2), and nowadays the glibc mmap() wrapper function invokes
> mmap2(2) with a suitably adjusted value for offset.
>
> I'm not sure if behaviour of glibc mmap() and syscall mmap2() would be
> same, but we should use glibc at most places which accounts for
> different architectures correctly. Maybe the differences were only
> present in case of mremap().
>
We shouldn't use glibc to test mseal, mseal is a security feature, and
an attacker can access syscall directly, so the test needs to test
with as little layer as possible.

I think there are two options:
1> switch everything to use __NR_mmap2
2> switch to __NR_mmap2 only for ARM.

I'm not sure which one is more appropriate though.

Thanks
-Jeff



> --
> BR,
> Muhammad Usama Anjum
>