linux-kernel - Re: [PATCH v6 2/3] random: introduce generic vDSO getrandom() implementation

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Date:   Thu, 24 Nov 2022 06:28:44 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        tglx@...utronix.de, linux-crypto@...r.kernel.org, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        Carlos O'Donell <carlos@...hat.com>
Subject: Re: [PATCH v6 2/3] random: introduce generic vDSO getrandom()
 implementation

* Jason A. Donenfeld:

> Hi Florian,
>
> On Wed, Nov 23, 2022 at 11:48:06AM +0100, Florian Weimer wrote:
>> * Jason A. Donenfeld:
>> 
>> >   static void *vgetrandom_alloc(size_t *num, size_t *size_per_each, unsigned int flags)
>> >   {
>> >     unsigned long ret = syscall(__NR_vgetrandom_alloc, num, size_per_each, flags);
>> >     return ret > -4096UL ? NULL : (void *)ret;
>> >   }
>> 
>> The traditional syscall function returns -1 on error and set errors, so
>> using unsing long and the 4096 is quite misleading.
>
> Not sure I have any idea at all whatsoever about what you're talking
> about. Firstly, the function you quoted is from the "sample userspace
> code" in the commit message, so it might not be code for the context you
> have in mind.

I'm talking about the syscall function that is available through
userspace via <sys/syscall.h>.

> Secondly, it's just doing the thing to figure out if the return value is
> an error value or a pointer. Were we in glibc, we'd write this as:
>
>     return INTERNAL_SYSCALL_ERROR_P(r) ? NULL : (void *) r;
>
> Right? And if you look at the expansion of that glibc macro, it's just:
>
>   #define INTERNAL_SYSCALL_ERROR_P(val) \
>     ((unsigned long int) (val) > -4096UL)
>
> So it looks like the same exact thing?

syscall already does internally (with a translation to -1, not NULL), so
the caller shouldn't do it again.  The userspace syscall function does
*not* return an error code.

Thanks,
Florian