[Bug libc/16919] recvmsg standard compliance

["joseph at codesourcery dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=16919

--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
Padding should use __glibc_reserved* names.

To make part of an existing field into padding like this, the following 
need to be true:

* If the kernel reads the field, all existing kernel versions supported by 
glibc already treat that part of the field as padding, so they will not 
misinterpret uninitialized data there.  If that's not the case, glibc 
needs to initialize the padding appropriately before passing the structure 
to the kernel.

* If the kernel writes the field, it gives an overflow error rather than 
treating that part of the field as something to which a significant value 
may be written.  If it treats it as significant, glibc needs to check in 
userspace for overflow and set an error in that case (just as the 
asm-generic implementations of 32-bit stat functions do).

* If user programs write the field, so that for old programs the high bits 
are significant but for new programs the high bits are padding, symbol 
versioning needs to be used to avoid misinterpreting values written there 
by existing binaries.

* If user programs read the field, so that old programs may interpret high 
bits written by glibc or the kernel as significant, those bits need 
initializing appropriately for the existing programs.

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