# 13. SGCheck: an experimental stack and global array overrun detector

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

## 13.1. Overview

SGCheck is a tool for finding overruns of stack and global arrays. It works by using a heuristic approach derived from an observation about the likely forms of stack and global array accesses.

## 13.2. SGCheck Command-line Options

There are no SGCheck-specific command-line options at present.

## 13.3. How SGCheck Works

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 it. 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 SGCheck 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, SGCheck 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 make multiple simultaneous calls to (e.g.) `memcpy` without mutual interference.

It is important to note that the association is done between a binary instruction and an array, the first time this binary instruction accesses an array during a function call. When the same instruction is executed again during the same function call, then SGCheck might report a problem, if these further executions are not accessing the same array. This technique causes several limitations in SGCheck, see Limitations.

## 13.4. Comparison with Memcheck

SGCheck and Memcheck are complementary: their capabilities do not overlap. Memcheck performs bounds checks and use-after-free checks for heap arrays. It also finds uses of uninitialised values created by heap or stack allocations. But it does not perform bounds checking for stack or global arrays.

SGCheck, on the other hand, does do bounds checking for stack or global arrays, but it doesn't do anything else.

## 13.5. 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.

- False negatives (missed errors): it follows from the description above (How SGCheck Works) 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 SGCheck uses that as the "example" of how subsequent accesses should behave.

  It also means that errors will not be found in an instruction executed only once (e.g. because this instruction is not in a loop, or the loop is executed only once).

- False positives (false errors): similarly, and more serious, it is clearly possible to write legitimate pieces of code which break the basic assumption upon which the checking algorithm depends. 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: SGCheck has to read 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, SGCheck 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: SGCheck runs slower than Memcheck. This is partly due to a lack of tuning, but partly due to algorithmic difficulties. 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. A redesign and reimplementation could potentially make it much faster.

- Coverage: Stack and global checking is fragile. If a shared object does not have debug information attached, then SGCheck 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 SGCheck accepts objects lacking debuginfo without comment. This is dangerous as it causes SGCheck to silently skip stack and global checking for such objects. It would be better to print a warning in such circumstances.

- Coverage: SGCheck does not check whether the areas read or written by system calls do overrun stack or global arrays. This would be easy to add.

- Platforms: the stack/global checks won't work properly on PowerPC, ARM or S390X 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 on ABIs that use a link register for function returns.

- Robustness: related to the previous point. Function call/exit tracking for X86 and 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.

## 13.6. 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.

- Add some heuristic filtering that removes obvious false positives. This would be easy to do. For example, an access transition from a heap to a stack object almost certainly isn't a bug and so should not be reported to the user.

## 13.7. 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.

- sg_main.c: Redesign and reimplement the basic checking algorithm. It could be done much faster than it is -- the current implementation isn't very good.

- 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.

- 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).