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Date: Wed, 27 Jan 2021 00:29:05 +0300
From: Andrey Melnikov <>
To: Rich Felker <>
Subject: Re: move __BYTE_ORDER definition to alltypes.h

вт, 26 янв. 2021 г. в 20:23, Rich Felker <>:
> On Tue, Jan 26, 2021 at 02:55:09PM +0300, Andrey Melnikov wrote:
> > Hi.
> >
> > Your commit 97d35a552ec5b6ddf7923dd2f9a8eb973526acea leads to
> > miscompile programs which rely on one of defines __LITTLE_ENDIAN or
> > Now, both unconditionally  defined when included stdarg.h and programs
> > which define __(BIG|LITTE)_ENDIAN itself - miscompiled. linux kernel
> > for example - it internally uses #if defined __BIG_ENDIAN and defines
> > it only  for BIGENDAIN arches.
> >
> > Any ideas?
> The conditionally-defined macros that on some archs tell you the
> endianness are __BIG_ENDIAN__ and __LITTLE_ENDIAN__ (note the final

Yes, defined by compiller.

> __) or other arch-specific macros. __BIG_ENDIAN and __LITTLE_ENDIAN
> (without the final __) have always been the possible values for
> __BYTE_ORDER from endian.h.

>From <endian.h> - its correct headers for this, not from <stdarg.h>.
<stdarg.h> is for va_* macros.

> In any case, all of these are in the
> reserved namespace and should not be defined by applications or
> inspected in any way other than a manner documented by the
> implementation.

Where is it reserved?

> How did this come up with the Linux kernel? AFAIK it uses -nostdinc
> and should not see the libc headers at all. But if that #ifdef is
> present in Linux it's probably a bug since it's contrary to all
> historical use of __BIG_ENDIAN...

Easy. Build an out-of-tree module that includes <linux/kernel.h> which
is include <stdarg.h> which is define all variant _ENDIAN macros,
later it include <asm/byteorder.h> which is include
<linux/byteorder/little_endian.h> in my case,
in <linux/byteorder/little_endian.h> (read
uapi/linux/byteorder/little_endian.h) we have defines:

#define __LITTLE_ENDIAN 1234

and later, include <linux/etherdevice.h> in module for
is_multicast_ether_addr() function:

static inline bool is_multicast_ether_addr(const u8 *addr)
        u32 a = *(const u32 *)addr;
        u16 a = *(const u16 *)addr;
#ifdef __BIG_ENDIAN
        return 0x01 & (a >> ((sizeof(a) * 8) - 8));
        return 0x01 & a;

And now, we have defined __LITTLE_ENDIAN 1234 (from kernel headers),
__BIG_ENDIAN 4321 (from stdarg header) - and code for BIG endian
compiled for LITTLE endian machide. Doh.

Please, return  (BIG|LITTLE|PDP)_ENDIAN defines back to <endian.h>.

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