This looks interesting, but I'm curious whether it will allow me to replace
all invocations of malloc. For example if my code calls a function defined
in some system library, and that function in turn calls malloc(), will it
call my interposed malloc?
Jeremy
> -----Original Message-----
> From: Chris Sarcone [mailto:email@hidden]
> Sent: Wednesday, March 05, 2008 6:03 PM
> To: Jeremy Todd
> Cc: email@hidden
> Subject: Re: vm_protect with VM_PROT_COPY not working on Leopard
>
> Jeremy --
>
> You might find this helpful:
>
> http://developer.apple.com/documentation/DeveloperTools/Concep
> tual/DynamicLibraries/DynamicLibraries.pdf
>
> Search for "interposing".
>
>
> -- Chris
>
> On Mar 5, 2008, at 2:44 PM, Jeremy Todd wrote:
>
>
> Hi,
>
> I'm trying to patch the malloc function, and my
> solution works under 10.4 but not 10.5. As I understand it, I
> need to call vm_protect() to get write access to the malloc
> function. It seems that malloc is in a system library which
> is shared across all processes, so I need the VM_PROT_COPY
> flag to enable copy-on-write semantics. Here's the code I'm using:
>
> int retval= vm_protect( mach_task_self(),
> reinterpret_cast<vm_address_t>(fnMalloc),
> MALLOC_HEADER_BYTES, false, VM_PROT_READ | VM_PROT_COPY );
> ...
> // Patch the malloc function with a jmp instruction
> fnMalloc[0]= 0xE9;
> *reinterpret_cast<int32*>(fnMalloc+1)=
> reinterpret_cast<int32>(pStubFunction) -
> reinterpret_cast<int32>(fnMalloc) - 5;
> ...
>
> I'm getting a segfault when I attempt to actually patch
> the malloc function under 10.5, but this works fine under
> 10.4. Does anyone why this might be failing, or have an idea
> about how I can patch the malloc function successfully under Leopard?
>
> In case anyone is wondering, I need to do this as part
> of our development process for audio plug-ins. These plug-ins
> should not call certain functions (including malloc) from the
> high-priority processing thread to avoid priority inversion
> and related problems. We have special debug builds which
> patch the malloc function, and if a malloc is detected in the
> processing thread, we get a very nice call stack in the
> debugger which makes it trivial to find the source of these problems.
>
> I've looked at other solutions for patching functions,
> and actually writing to the code page seems like the cleanest
> way to go. This code is only for internal use so we don't
> even need to worry about reverting the system to its original
> state, etc., as long as this helps us track down problems.
>
> Note also that a previous post addressed this issue:
>
> http://lists.apple.com/archives/darwin-kernel/2005/Feb/msg00045.html
> but I didn't have any luck with the DYLD_SUFFIX
> solution, and the copy-on-write solution given there is what
> I'm posting about.
>
> Anyway I hope this is clear and any assistance would be
> most appreciated.
>
> Regards,
> Jeremy
>
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