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Date: Mon, 16 Apr 2018 00:40:06 -0400
From: Rich Felker <>
Subject: Re: tcmalloc compatibility

On Mon, Apr 16, 2018 at 06:19:24AM +0200, Markus Wichmann wrote:
> On Sun, Apr 15, 2018 at 01:52:10PM +0200, ardi wrote:
> > On Tue, Apr 10, 2018 at 11:17 PM, Szabolcs Nagy <> wrote:
> > >
> > > then the wrappers with dlsym(RTLD_NEXT,sym) would not work.
> > > (malloc checkers, valgrind, sanitizers etc all do it)
> > 
> > I've been using ElectricFence, as my only memory debugger since 1996
> > or so; mostly with the libc of commercial Unices, but also with glibc
> > in Linux, and with the OSX libc. I never considered I could run into
> > the issues commented in this thread, and in fact I never faced these
> > issues and it always worked as expected (however, I must admit I only
> > use multithreading for accelerating clearly isolated math-intensive
> > loops that don't call malloc-related functions from inside the loop).
> > 
> > Said this, when I'm linking with ElectricFence, my brain has the "hack
> > mode flag" ON (I mean, I always had the feeling that I'm working with
> > a temporary hack that can fail whenever my link line contains -lefence
> > , and I'm aware that things can go wrong --I didn't consider thread
> > safety, but anyway I know ElectricFence can fail if the OS syscalls
> > that allocate protected memory at buffer ends change their behaviour
> > in newer versions, or if there's some OS/CPU-dependent subtlety with
> > alignment, etc...)
> > 
> > I've not tried to use ElectricFence with musl yet... but reading this,
> > can I suppose it won't work? Is there any "hack mode ON" procedure
> > (yet easy) that would allow to use ElectricFence (assuming
> > non-threaded code, which is always my case).
> > 
> > I agree with your commitment to correctness, and I'm not asking for a
> > safe and guaranteed implementation of function interposition, just
> > that sometimes I need to break my binaries to make them crash hard as
> > soon as pointer accesses a byte it shouldn't access.
> > 
> > Cheers,
> > 
> > ardi
> So long as you refrain from using dynamic linking (because of the memory
> donation)

This is only a small part of the reason you can't use dynamic linking.
The other big part is that references in are bound at
link time, so functions like getline, open_memstream, strdup, etc.
will return pointers that won't be valid for you to free.

> and calloc() and memalign() (and posix_memalign()) are unused
> or overloaded, you should be fine. Both of these functions use the
> internal bookkeeping of musl's malloc. calloc() uses it to figure out if
> a chunk was mmapped (in which case no initialization is necessary), and
> memalign() uses it to construct a second chunk header to cause the
> returned pointer to be aligned.

Yes, but this rule always applies for interposing, with any
implementation. It's not musl-specific.

> Most of the questioning here arose from that first part. Those are the
> two big problems, actually, we need an interface to donate memory to the
> malloc implementation,

This isn't needed. It's fine for donation to donate to the internal
(unused) implementation if malloc is interposed, or for donation not
to happen at all. I don't think it's a good idea to create a public
interposable API for donation.

The big thing that does need to happen is getting rid of the call to
free() to do the donation, which is unsafe/incorrect if it's
interposed. Alexander Monakov's patch (which looks ok to commit with
minor changes described in the thread) should fix that.

> and the malloc implementation needs to provide
> all of the hairier functions like memalign(). And we currently have no
> way of enforcing either of these.

A way to enforce this was discussed earlier in the thread, so it looks


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