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bug#23760: 25.0.95; emacs 25.0.95 doesn't build with glibc-2.23.90

From: Florian Weimer
Subject: bug#23760: 25.0.95; emacs 25.0.95 doesn't build with glibc-2.23.90
Date: Mon, 20 Jun 2016 12:15:40 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1

On 06/20/2016 12:04 PM, Paul Eggert wrote:
On 06/20/2016 11:21 AM, Florian Weimer wrote:

The usual mechanism for deprecation and removal of an API does not
work if the symbol is interposed because it will be unversioned, and
unversioned symbols preempt versioned symbols.  As a result, even if
the symbol is a compat symbol, you can produce new binaries which use
the removed API.

True, but in this particular case Emacs is replacing malloc as well as
__malloc_initialize_hook etc., so I don't see a problem. Although new
Emacs binaries will still use the removed API, they will also support
the removed API.

You need just one linked DSOs which somehow manages to call a function in the glibc malloc implementation, and interesting things will happen.

What *could* be a problem is if the new glibc malloc API supplies
symbols that Emacs does not supply, and if other parts of the new glibc
use these symbols. But I don't see this happening either (and if it did
happen, poisoning __malloc_initialize_hook wouldn't fix it).

We already have this problem with malloc_usable_size, and perhaps some of the aligned allocation functions.

This reminds me of this glibc bug, which I've put on my list to fix:


I think after that, at least glibc will be interposition-clean.

Perhaps poisoning __malloc_initialize_hook helps for some theoretical
applications, but for Emacs I don't see how it is a win.

I'm worried that Emacs developers decide to ignore the API removal and keep using glibc malloc and the malloc_set_state function it provides. If we can turn the latter into a compatibility symbol during this development cycle, that would go a long way towards addressing my concern.


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