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[John Criswell <criswell AT illinois.edu>]

On 11/22/2011 6:48 PM, Matthew Wala wrote:
> Why does SAFECode allow malloc(0) but disallows alloca(0)?

Hrm.  I'm not sure if this is really what SAFECode *should* do, but I 
can think of some reasons why it does it.

I think malloc(0) does occur in some programs, and SAFECode could easily 
handle it by changing the allocation size to 1 in the poolalloc() 
function back when it was always using the pool allocator run-time.  
This cannot be done with allocas since alloca is an LLVM instruction and 
emitted straight to machine code.

It may also be the case that malloc(0) is correct (return a pointer to 
zero-sized storage) while alloca(0) is not, but I'd have to check the 
LLVM docs to be certain.

I think the correct thing to do now is to first determine whether 
malloc(0) and alloca(0) are allowed and, if so, permit the 
pool_register() and pool_register_stack() (and their variants) to 
register zero-sized objects.

That said, this is extremely low priority unless you're seeing a program 
that is breaking because of it.  Unless SAFECode is breaking a correct 
program, you should just file a bug report, and we'll get back to it 
when we have time.  Testing SAFECode out on larger programs is higher 
priority at present as well as fixing more serious bugs such as PR#11414 
and PR#11423.

-- John T.


> Matt
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