## 10. Ptrcheck: an experimental heap, stack and global array overrun detector

To use this tool, you must specify `--tool=exp-ptrcheck` on the Valgrind command line.

## 10.1. Overview

Ptrcheck is a tool for finding overruns of heap, stack and global arrays. Its functionality overlaps somewhat with Memcheck's, but it is able to catch invalid accesses in a number of cases that Memcheck would miss. A detailed comparison against Memcheck is presented below.

Ptrcheck is composed of two almost completely independent tools that have been glued together. One part, in `h_main.[ch]`, checks accesses through heap-derived pointers. The other part, in `sg_main.[ch]`, checks accesses to stack and global arrays. The remaining files `pc_{common,main}.[ch]`, provide common error-management and coordination functions, so as to make it appear as a single tool.

The heap-check part is an extensively-hacked (largely rewritten) version of the experimental "Annelid" tool developed and described by Nicholas Nethercote and Jeremy Fitzhardinge. The stack- and global- check part uses a heuristic approach derived from an observation about the likely forms of stack and global array accesses, and, as far as is known, is entirely novel.

## 10.2. Ptrcheck Command-line Options

Ptrcheck-specific command-line options are:

`--enable-sg-checks=no|yes [default: yes]`

  By default, Ptrcheck checks for overruns of stack, global and heap arrays. With `--enable-sg-checks=no`, the stack and global array checks are omitted, and only heap checking is performed. This can be useful because the stack and global checks are quite expensive, so omitting them speeds Ptrcheck up a lot.

`--partial-loads-ok=<yes|no> [default: no]`

  This option has the same meaning as it does for Memcheck.

  Controls how Ptrcheck handles word-sized, word-aligned loads which partially overlap the end of heap blocks -- that is, some of the bytes in the word are validly addressable, but others are not. When `yes`, such loads do not produce an address error. When `no` (the default), loads from partially invalid addresses are treated the same as loads from completely invalid addresses: an illegal heap access error is issued.

  Note that code that behaves in this way is in violation of the the ISO C/C++ standards, and should be considered broken. If at all possible, such code should be fixed. This option should be used only as a last resort.

## 10.3. How Ptrcheck Works: Heap Checks

Ptrcheck can check for invalid uses of heap pointers, including out of range accesses and accesses to freed memory. The mechanism is however completely different from Memcheck's, and the checking is more powerful.

For each pointer in the program, Ptrcheck keeps track of which heap block (if any) it was derived from. Then, when an access is made through that pointer, Ptrcheck compares the access address with the bounds of the associated block, and reports an error if the address is out of bounds, or if the block has been freed.

Of course it is rarely the case that one wants to access a block only at the exact address returned by `malloc` et al. Ptrcheck understands that adding or subtracting offsets from a pointer to a block results in a pointer to the same block.

At a fundamental level, this scheme works because a correct program cannot make assumptions about the addresses returned by `malloc` et al. In particular it cannot make any assumptions about the differences in addresses returned by subsequent calls to `malloc` et al. Hence there are very few ways to take an address returned by `malloc`, modify it, and still have a valid address. In short, the only allowable operations are adding and subtracting other non-pointer values. Almost all other operations produce a value which cannot possibly be a valid pointer.

## 10.4. How Ptrcheck Works: Stack and Global Checks

When a source file is compiled with `-g`, the compiler attaches DWARF3 debugging information which describes the location of all stack and global arrays in the file.

Checking of accesses to such arrays would then be relatively simple, if the compiler could also tell us which array (if any) each memory referencing instruction was supposed to access. Unfortunately the DWARF3 debugging format does not provide a way to represent such information, so we have to resort to a heuristic technique to approximate the same information. The key observation is that if a memory referencing instruction accesses inside a stack or global array once, then it is highly likely to always access that same array.

To see how this might be useful, consider the following buggy fragment:

       { int i, a[10];  // both are auto vars
         for (i = 0; i <= 10; i++)
            a[i] = 42;
       }

At run time we will know the precise address of `a[]` on the stack, and so we can observe that the first store resulting from `a[i] = 42` writes `a[]`, and we will (correctly) assume that that instruction is intended always to access `a[]`. Then, on the 11th iteration, it accesses somewhere else, possibly a different local, possibly an un-accounted for area of the stack (eg, spill slot), so Ptrcheck reports an error.

There is an important caveat.

Imagine a function such as `memcpy`, which is used to read and write many different areas of memory over the lifetime of the program. If we insist that the read and write instructions in its memory copying loop only ever access one particular stack or global variable, we will be flooded with errors resulting from calls to `memcpy`.

To avoid this problem, Ptrcheck instantiates fresh likely-target records for each entry to a function, and discards them on exit. This allows detection of cases where (e.g.) `memcpy` overflows its source or destination buffers for any specific call, but does not carry any restriction from one call to the next. Indeed, multiple threads may be multiple simultaneous calls to (e.g.) `memcpy` without mutual interference.

## 10.5. Comparison with Memcheck

Memcheck does not do any access checks for stack or global arrays, so the presence of those in Ptrcheck is a straight win. (But see "Limitations" below).

Memcheck and Ptrcheck use different approaches for checking heap accesses. Memcheck maintains bitmaps telling it which areas of memory are accessible and which are not. If a memory access falls in an unaccessible area, it reports an error. By marking the 16 bytes before and after an allocated block unaccessible, Memcheck is able to detect small over- and underruns of the block. Similarly, by marking freed memory as unaccessible, Memcheck can detect all accesses to freed memory.

Memcheck's approach is simple. But it's also weak. It can't catch block overruns beyond 16 bytes. And, more generally, because it focusses only on the question "is the target address accessible", it fails to detect invalid accesses which just happen to fall within some other valid area. This is not improbable, especially in crowded areas of the process' address space.

Ptrcheck's approach is to keep track of pointers derived from heap blocks. It tracks pointers which are derived directly from calls to `malloc` et al, but also ones derived indirectly, by adding or subtracting offsets from the directly-derived pointers. When a pointer is finally used to access memory, Ptrcheck compares the access address with that of the block it was originally derived from, and reports an error if the access address is not within the block bounds.

Consequently Ptrcheck can detect any out of bounds access through a heap-derived pointer, no matter how far from the original block it is.

A second advantage is that Ptrcheck is better at detecting accesses to blocks freed very far in the past. Memcheck can detect these too, but only for blocks freed relatively recently. To detect accesses to a freed block, Memcheck must make it inaccessible, hence requiring a space overhead proportional to the size of the block. If the blocks are large, Memcheck will have to make them available for re-allocation relatively quickly, thereby losing the ability to detect invalid accesses to them.

By contrast, Ptrcheck has a constant per-block space requirement of four machine words, for detection of accesses to freed blocks. A freed block can be reallocated immediately, yet Ptrcheck can still detect all invalid accesses through any pointers derived from the old allocation, providing only that the four-word descriptor for the old allocation is stored. For example, on a 64-bit machine, to detect accesses in any of the most recently freed 10 million blocks, Ptrcheck will require only 320MB of extra storage. Achieving the same level of detection with Memcheck is close to impossible and would likely involve several gigabytes of extra storage.

Having said all that, remember that Memcheck performs uninitialised value checking, invalid and mismatched free checking, overlap checking, and leak checking, none of which Ptrcheck do. Memcheck has also benefitted from years of refinement, tuning, and experience with production-level usage, and so is much faster than Ptrcheck as it currently stands.

Consequently we recommend you first make your programs run Memcheck clean. Once that's done, try Ptrcheck to see if you can shake out any further heap, global or stack errors.

## 10.6. Limitations

This is an experimental tool, which relies rather too heavily on some not-as-robust-as-I-would-like assumptions on the behaviour of correct programs. There are a number of limitations which you should be aware of.

- Heap checks: Ptrcheck can occasionally lose track of, or become confused about, which heap block a given pointer has been derived from. This can cause it to falsely report errors, or to miss some errors. This is not believed to be a serious problem.

- Heap checks: Ptrcheck only tracks pointers that are stored properly aligned in memory. If a pointer is stored at a misaligned address, and then later read again, Ptrcheck will lose track of what it points at. Similar problem if a pointer is split into pieces and later reconsitituted.

- Heap checks: Ptrcheck needs to "understand" which system calls return pointers and which don't. Many, but not all system calls are handled. If an unhandled one is encountered, Ptrcheck will abort. Fortunately, adding support for a new syscall is very easy.

- Stack checks: It follows from the description above (How Ptrcheck Works: Stack and Global Checks) that the first access by a memory referencing instruction to a stack or global array creates an association between that instruction and the array, which is checked on subsequent accesses by that instruction, until the containing function exits. Hence, the first access by an instruction to an array (in any given function instantiation) is not checked for overrun, since Ptrcheck uses that as the "example" of how subsequent accesses should behave.

- Stack checks: Similarly, and more serious, it is clearly possible to write legitimate pieces of code which break the basic assumption upon which the stack/global checking rests. For example:

        { int a[10], b[10], *p, i;
          for (i = 0; i < 10; i++) {
             p = /* arbitrary condition */  ? &a[i]  : &b[i];
             *p = 42;
          }
        }

  In this case the store sometimes accesses `a[]` and sometimes `b[]`, but in no cases is the addressed array overrun. Nevertheless the change in target will cause an error to be reported.

  It is hard to see how to get around this problem. The only mitigating factor is that such constructions appear very rare, at least judging from the results using the tool so far. Such a construction appears only once in the Valgrind sources (running Valgrind on Valgrind) and perhaps two or three times for a start and exit of Firefox. The best that can be done is to suppress the errors.

- Performance: the stack/global checks require reading all of the DWARF3 type and variable information on the executable and its shared objects. This is computationally expensive and makes startup quite slow. You can expect debuginfo reading time to be in the region of a minute for an OpenOffice sized application, on a 2.4 GHz Core 2 machine. Reading this information also requires a lot of memory. To make it viable, Ptrcheck goes to considerable trouble to compress the in-memory representation of the DWARF3 data, which is why the process of reading it appears slow.

- Performance: Ptrcheck runs slower than Memcheck. This is partly due to a lack of tuning, but partly due to algorithmic difficulties. The heap-check side is potentially quite fast. The stack and global checks can sometimes require a number of range checks per memory access, and these are difficult to short-circuit (despite considerable efforts having been made).

- Coverage: the heap checking is relatively robust, requiring only that Ptrcheck can see calls to `malloc` et al. In that sense it has debug-info requirements comparable with Memcheck, and is able to heap-check programs even with no debugging information attached.

  Stack/global checking is much more fragile. If a shared object does not have debug information attached, then Ptrcheck will not be able to determine the bounds of any stack or global arrays defined within that shared object, and so will not be able to check accesses to them. This is true even when those arrays are accessed from some other shared object which was compiled with debug info.

  At the moment Ptrcheck accepts objects lacking debuginfo without comment. This is dangerous as it causes Ptrcheck to silently skip stack and global checking for such objects. It would be better to print a warning in such circumstances.

- Coverage: Ptrcheck checks that the areas read or written by system calls do not overrun heap blocks. But it doesn't currently check them for overruns stack and global arrays. This would be easy to add.

- Platforms: the stack/global checks won't work properly on any PowerPC platforms, only on x86 and amd64 targets. That's because the stack and global checking requires tracking function calls and exits reliably, and there's no obvious way to do it with the PPC ABIs. (In comparison, with the x86 and amd64 ABIs this is relatively straightforward.)

- Robustness: related to the previous point. Function call/exit tracking for x86/amd64 is believed to work properly even in the presence of longjmps within the same stack (although this has not been tested). However, code which switches stacks is likely to cause breakage/chaos.

## 10.7. Still To Do: User-visible Functionality

- Extend system call checking to work on stack and global arrays.

- Print a warning if a shared object does not have debug info attached, or if, for whatever reason, debug info could not be found, or read.

## 10.8. Still To Do: Implementation Tidying

Items marked CRITICAL are considered important for correctness: non-fixage of them is liable to lead to crashes or assertion failures in real use.

- h_main.c: make N_FREED_SEGS command-line configurable.

- sg_main.c: Improve the performance of the stack / global checks by doing some up-front filtering to ignore references in areas which "obviously" can't be stack or globals. This will require using information that m_aspacemgr knows about the address space layout.

- h_main.c: get rid of the last_seg_added hack; add suitable plumbing to the core/tool interface to do this cleanly.

- h_main.c: move vast amounts of arch-dependent ugliness (get_IntRegInfo et al) to its own source file, a la mc_machine.c.

- h_main.c: make the lossage-check stuff work again, as a way of doing quality assurance on the implementation.

- h_main.c: schemeEw_Atom: don't generate a call to nonptr_or_unknown, this is really stupid, since it could be done at translation time instead.

- CRITICAL: h_main.c: h_instrument (main instrumentation fn): generate shadows for word-sized temps defined in the block's preamble. (Why does this work at all, as it stands?)

- sg_main.c: fix compute_II_hash to make it a bit more sensible for ppc32/64 targets (except that sg_ doesn't work on ppc32/64 targets, so this is a bit academic at the moment).