Hi James,

I looked at the code of tcmalloc, but I didn't find any of the problems you mentioned in the implementation of malloc_usable_size (see: https://github.com/google/tcmalloc/blob/9179bb884848c30616667ba129bcf9afee114c32/tcmalloc/tcmalloc.cc#L1099 ).

On the contrary, similar to musl, tcmalloc also directly uses the return value of malloc_usable_size in its realloc implementation to determine whether memory needs to be reallocated: https://github.com/google/tcmalloc/blob/9179bb884848c30616667ba129bcf9afee114c32/tcmalloc/tcmalloc.cc#L1499

I think this is enough to show that the return value of malloc_usable_size in tcmalloc is accurate and reliable, otherwise its own realloc will cause a segment fault.

Thanks :-)

--

   Best Regards
  BaiYang
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From: James Y Knight
Date: 2022-09-19 21:53
To: musl
CC: Florian Weimer; baiyang
Subject: Re: [musl] The heap memory performance (malloc/free/realloc) is significantly degraded in musl 1.2 (compared to 1.1)
Indeed. RedHat mentioned that problem in their recent post about _FORTIFY_SOURCE=3, here
https://developers.redhat.com/articles/2022/09/17/gccs-new-fortification-level

"""
_FORTIFY_SOURCE=3 revealed another pattern. Applications such as systemd used malloc_usable_size to determine available space in objects and then used the residual space. The glibc manual discourages this type of usage, dictating that malloc_usable_size is for diagnostic purposes only. But applications use the function as a hack to avoid reallocating buffers when there is space in the underlying malloc chunk. The implementation of malloc_usable_size needs to be fixed to return the allocated object size instead of the chunk size in non-diagnostic use. Alternatively, another solution is to deprecate the function. But that is a topic for discussion by the glibc community.
"""

On Mon, Sep 19, 2022 at 9:47 AM Rich Felker <dalias@libc.org> wrote:
On Mon, Sep 19, 2022 at 02:36:41PM +0200, Florian Weimer wrote:
> * Szabolcs Nagy:
> 
> > unlike musl those implementations don't return exact size nor have the
> > same security and memory fragmentation guarantees, so bad comparision.
> >
> > tcmalloc:
> >   // Returns the actual number N of bytes reserved by tcmalloc for the pointer
> >   // p.  This number may be equal to or greater than the number of bytes
> >   // requested when p was allocated.
> >   //
> >   // This function is just useful for statistics collection.  The client must
> >   // *not* read or write from the extra bytes that are indicated by this call.
> >
> > jemalloc:
> >       <para>The <function>malloc_usable_size()</function> function
> >       returns the usable size of the allocation pointed to by
> >       <parameter>ptr</parameter>.  The return value may be larger than the size
> >       that was requested during allocation.  The
> >       <function>malloc_usable_size()</function> function is not a
> >       mechanism for in-place <function>realloc()</function>; rather
> >       it is provided solely as a tool for introspection purposes.  Any
> >       discrepancy between the requested allocation size and the size reported
> >       by <function>malloc_usable_size()</function> should not be
> >       depended on, since such behavior is entirely implementation-dependent.
> 
> These implementations are buggy or at least mis-documented.  The
> interface contract is clearly that for that particular object, the extra
> bytes in the allocation are available for reading and writing.  It is
> not guaranteed that the allocator will always provide the same number of
> extra bytes for the same requested size, but they must be there for the
> allocation being examined.  It's even in the name of the function!

I'm not sure I understand what you're saying, but the core problem
that really can't be solved is potential discrepancy between the
malloc implementation's idea of usable and the compiler's. For
example:

        char *p = malloc(1);
        if (malloc_usable_size(p)>1) p[1] = 42;

will cause a compiler that's actively detecting UB to abort the
program when malloc_usable_size returns a value larger than 1.

Rich