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Subject: Re: [RFC v2] bpf.2: Use standard types and attributes
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Zack Weinberg <zackw@panix.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
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        "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
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        glibc <libc-alpha@sourceware.org>, GCC <gcc-patches@gcc.gnu.org>,
        bpf <bpf@vger.kernel.org>,
        Joseph Myers <joseph@codesourcery.com>,
        David Laight <David.Laight@aculab.com>
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From:   "Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>
Message-ID: <0d9a795a-7c6a-3889-af31-2223dc216d15@gmail.com>
Date:   Tue, 4 May 2021 21:59:06 +0200
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Hi Florian,

On 5/4/21 9:45 PM, Florian Weimer wrote:
> * Alejandro Colomar:
> 
>> The thing is, in all of those threads, the only reasons to avoid
>> <stdint.h> types in the kernel (at least, the only explicitly
>> mentioned ones) are (a bit simplified, but this is the general idea of
>> those threads):
>>
>> * Possibly breaking something in such a big automated change.
>> * Namespace collision with userspace (the C standard allows defining
>>    uint32_t for nefarious purposes as long as you don't include
>>   <stdint.h>.   POSIX prohibits that, though)
>> * Uglier
> 
> __u64 can't be formatted with %llu on all architectures.  That's not
> true for uint64_t, where you have to use %lu on some architectures to
> avoid compiler warnings (and technically undefined behavior).  There are
> preprocessor macros to get the expected format specifiers, but they are
> clunky.  I don't know if the problem applies to uint32_t.  It does
> happen with size_t and ptrdiff_t on 32-bit targets (both vary between
> int and long).
> 

Hmmm, that's interesting.  It looks like Linux always uses long long for 
64 bit types, while glibc uses 'long' as long as it's possible, and only 
uses 'long long' when necessary.  Assignment is still 100% valid both 
ways and binary compatibility also 100% (AFAIK), given they're the same 
length and signedness, but pointers are incompatible.  That's something 
to note, even though in this case there are no pointers involved, so no 
incompatibilities.  Maybe the kernel and glibc could use the same rules 
to improve compatibility, but that's out of the scope of this.

Thanks,

Alex


-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/