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Date:   Tue, 23 May 2023 16:25:45 +0300
From:   Alexey Izbyshev <izbyshev@ispras.ru>
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Florent Revest <revest@chromium.org>,
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Subject: Re: [PATCH v2 3/5] mm: Make PR_MDWE_REFUSE_EXEC_GAIN an unsigned long
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On 2023-05-23 16:07, Catalin Marinas wrote:
> On Tue, May 23, 2023 at 11:12:37AM +0200, David Hildenbrand wrote:
>> Also, how is passing "0"s to e.g., PR_GET_THP_DISABLE reliable? We 
>> need arg2
>> -> arg5 to be 0. But wouldn't the following also just pass a 0 "int" ?
>> 
>> prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0)
>> 
>> I'm easily confused by such (va_args) things, so sorry for the dummy
>> questions.
> 
> Isn't the prctl() prototype in the user headers defined with the first
> argument as int while the rest as unsigned long? At least from the man
> page:
> 
> int prctl(int option, unsigned long arg2, unsigned long arg3,
> 	  unsigned long arg4, unsigned long arg5);
> 
> So there are no va_args tricks (which confuse me as well).
> 
I have explicitly mentioned the problem with man pages in my response to 
David[1]. Quoting myself:

> This stuff *is* confusing, and note that Linux man pages don't even 
> tell
that prctl() is actually declared as a variadic function (and for
ptrace() this is mentioned only in the notes, but not in its signature).

The reality:

* glibc: 
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/sys/prctl.h;h=821aeefc1339b35210e8918ecfe9833ed2792626;hb=glibc-2.37#l42

* musl: 
https://git.musl-libc.org/cgit/musl/tree/include/sys/prctl.h?h=v1.2.4#n180

Though there is a test in the kernel that does define its own prototype, 
avoiding the issue: 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/sched/cs_prctl_test.c?h=v6.3#n77

Thanks,
Alexey

[1] 
https://lore.kernel.org/lkml/7c572622c0d8e283fc880fe3f4ffac27@ispras.ru//lkml/7c572622c0d8e283fc880fe3f4ffac27@ispras.ru

> Any int passed to arg[2-5] would be converted by the compiler to an
> unsigned long before being passed to the kernel. So I think the change
> in this patch is harmless as the conversion is happening anyway.
> 
> (well, unless I completely missed what the problem is)