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Date: Thu, 30 May 2019 13:52:39 -0400
From: sva sva <>
Subject: Re: How to get base address of heap arenas

Correction: I currently only follow the first malloc back to the top chunk
and would NOT like to follow every malloc due to the obvious performance
impact. This, however, fails when there are multiple non contiguous heap

On Thu, May 30, 2019 at 1:49 PM sva sva <> wrote:

> Indeed your replies verified my understanding.
> Here's what I want to do:
> I have a C library that does malloc interposition and adds signatures to
> header and footer of chunks by allocating extra bytes before and after it.
> It also does a number of other things. Then I have some C code which is
> supposed to walk the entire heap to that particular process and spit out
> all the heap signatures making sure there are no scribbled ones.
> What I am doing right now is that whenever I get a malloc/calloc/etc.,
> which is of course overloaded, I follow the returned pointer all the way
> back to reach the top chunk (prev size = 0). This would fail when I get
> chunks in different memory regions that are not contiguous. So I need to
> somehow keep track of the contiguous heap areas and get the top chunk for
> all of those to start my heap walk. Last thing I want to do is to patch
> musl's libc.
> Thanks
> Vahid
> On Thu, May 30, 2019 at 10:02 AM Rich Felker <> wrote:
>> On Thu, May 30, 2019 at 03:27:24PM +0200, Szabolcs Nagy wrote:
>> > * sva sva <> [2019-05-30 08:59:46 -0400]:
>> > > I am writing a heap walk program in C and would like to know if there
>> is
>> > > anything like the concept of arenas in musl. Basically, I need to
>> have a
>> > > pointer to the base address of all my allocated heaps. Unfortunately
>> > > inspecting the musl code I found none.
>> >
>> > there is no such concept as "heap arena" visible to user
>> > code, so almost surely you don't "need a pointer to the
>> > base address" of it.
>> >
>> > try to describe what exactly you want to do (not in terms
>> > of libc internals, but in terms that make sense for user
>> > code)
>> >
>> > (do you want to get all memory mappings? -> try /proc/self/maps)
>> > (do you want to track malloc behaviour? -> try malloc interposition)
>> > (etc)
>> To further clarify, the current allocator implementation has no global
>> view of "the heap". It sees only free chunks and the headers or
>> footers of the immediately adjacent-in-address-space allocated chunks.
>> It's likely that the future replacement will have global tracking that
>> further helps ensure integrity against heap corruption attacks, but it
>> won't be a public API or something necessarily stable between
>> versions. If you want a debugging malloc, you need to interpose one;
>> for the past few releases, musl has supported malloc interposition.
>> Rich

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