Jordan,<br><br>Thanks for the feedback. <br><br>I have a few questions about your comments. My first impression when I looked at your examples was 'why would that not work'? Originally I believed that some state regarding the allocate function would be remembered, and when it was invoked the static checking framework would mark that the pointer returned to the caller would always have an offset of 1. I am not sure that is true though. And further, even if it was, posting a warning at the free inside the deallocate function would not make sense, as the pointer passed to deallocate could have multiple possible offsets depending on what the program was doing. To be effective, the warning would need to be when deallocate was called, which would be much tricker.<br>
<br>The more I think about this, the more I am getting lost in some of the details. When deallocate is analyzed, nothing is really known about what memoryBlock really represents. The caller could be passing malloc'd memory or something else. I like your suggestion about making sure that the memoryBlock actually has known malloc information. This would limit the checker to only using malloc and free, and only within the same function. However, I think that is what <span class="name">memory.LeakPtrValChanged </span>is supposed to accomplish anyway.<br>
<br>A final point I am not clear on is how to determine if a MemRegion has malloc information. I see where the ReportBadFree function in MallocChecker.cpp is finding the base MemRegion, and also determining if the region is generated from alloca. How would one know if it was generated from malloc instead? Maybe if the MemRegion was a HeapSpaceRegion?<br>
<br>Also, thanks for pointing out the coding standards. I will follow them more closely in the future.<br><br>- Branden<br><br>
<br><div class="gmail_quote">On Mon, Dec 17, 2012 at 12:59 PM, Jordan Rose <span dir="ltr"><<a href="mailto:jordan_rose@apple.com" target="_blank">jordan_rose@apple.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="word-wrap:break-word"><div>Hi, Brandon. It's a good idea, but unfortunately it has some problems. In C, it's totally legal to do this:</div><div><br></div><div><blockquote type="cite">int *allocate(size_t size) {</blockquote>
</div><div><blockquote type="cite"> int *memoryBlock = (int *)malloc(size + sizeof(int));</blockquote><blockquote type="cite"> *memoryBlock = SECRET_CODE;</blockquote><blockquote type="cite"> return &memoryBlock[1];</blockquote>
<blockquote type="cite">}</blockquote><br></div><div><blockquote type="cite">void deallocate(int *memoryBlock) {</blockquote><blockquote type="cite"> assert(memoryBlock[-1] == SECRET_CODE);</blockquote><blockquote type="cite">
free(&memoryBlock[-1]);</blockquote><blockquote type="cite">}</blockquote></div><div><br></div><div>I'm not sure of the best way to solve this. IIRC, by default the region '*memoryBlock' will be a SymbolicRegion backed by a RegionValueSymbol, but if 'deallocate' has been inlined the backing symbol could be a DerivedRegionSymbol or a ConjuredSymbol instead. So it'd be very hard to differentiate these cases without actually seeing the call to allocate().</div>
<div><br></div><div>What you could try is seeing if the base region already has malloc information. That will miss some true bugs, but it should also drastically lower the rate of false positives, since we'll only be warning about regions we <i>know</i> can be freed.</div>
<div><br></div><div>As far as the patch itself, your logic seems reasonable, but your style doesn't match the rest of the file or the <a href="http://llvm.org/docs/CodingStandards.html" target="_blank">LLVM Coding Standards</a>. In particular, please put a space after 'if', put operators at the end of the previous line instead of the start of the next line, and fit your lines in 80 columns. I'd also prefer '!offset.hasSymbolicOffset()' over 'offset.hasSymbolicOffset() == false'.</div>
<div><br></div><div>You'll probably want more test cases: cases where the input parameter does <i>not</i> come from malloc but has an offset, and at least one case where the input parameter comes from outside the function and has an offset.</div>
<div><br></div><div>Thanks for working on this!</div><div>Jordan</div><div><br></div><div><br></div><br><div><div><div><div>On Dec 15, 2012, at 21:58 , Branden Archer <<a href="mailto:b.m.archer4@gmail.com" target="_blank">b.m.archer4@gmail.com</a>> wrote:</div>
<br></div></div><blockquote type="cite"><div><div>I have recently started looking into clang, and was interested in participating. After taking a look at the potential projects, the static checking functionality seemed interesting. Specifically, I have taken a look at the <span>checker "memory.LeakPtrValChanged</span>" mentioned on the list of potential checkers page.<br>
<br>Warning: As this is my first attempt at hacking clang, I may have gone a different route than someone with more experience in the project. If something in my description or patch seems out of place, please let me know!<br>
<br>From the description, the proposed <span>memory.LeakPtrValChanged checker was to only consider a pointer to newly allocated data losing its original
value. Through some investigation, I find that MemRegion objects which track pointers to memory allocations can also maintain any offset currently applied to the pointer. Using this information, the checker can reason about invalidated pointers beyond being 'newly allocated'. For example, the following case can be caught:<br>
<br>int * x = malloc(sizeof(int));<br>x += 1;<br>free(x);<br><br>However, the following is valid:<br></span><br><span><span>int * x = malloc(sizeof(int));<br>x += 1;<br>free(x-1);<br><br></span>The attached patch uses the RegionOffset of freed malloc allocations to determine if the freed pointer has a non-zero offset, and post a warning in this case. If the offset is symbolic (and thus not known to be non-zero), no warning is posted. There are tests included to verify the proposed changes. <br>
<br>Note that <span>memory.LeakPtrValChanged mentioned checking both malloc/free and new/delete, but this patch only considers malloc/free.<br><br>Please let me know if the attached patch is appropriate, or if it is missing something or there is another solution which may be a better fit.<br>
<br>- Branden<br></span></span><span><br></span>
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