<br><br><div class="gmail_quote">On Wed, Aug 15, 2012 at 11:47 AM, Kostya Serebryany <span dir="ltr"><<a href="mailto:kcc@google.com" target="_blank">kcc@google.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div class="gmail_quote"><div><div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><br></div><div><div>3) Handling and reporting of undefined behavior, once caught. The sanitizers produce decent information (which is both useful and detailed), but it could be prettier. </div>
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<div><br></div></div></div><div>Can you be more specific? </div></div></blockquote><div><br></div></div></div><div>Sure. Here's a few things which I think could be improved:</div><div><br></div><div>* The point at which the error is trapped doesn't include a column number. (I'd also like a code snippet with a caret, where possible, but there are tradeoffs to be carefully considered there...)</div>
</div></blockquote><div><br></div></div><div>Agree. In some cases this could be very valuable, e.g. when reporting a bug on expressions like a->b->c = x->y->z; </div><div>With your help we may be able to implement this as part of our symbolizer work.</div>
</div></blockquote><div><br></div><div>FYI column numbers are already there in DWARF debug info, and we don't have to do anything specific to fetch them. They are missing</div><div>in ASan error reports because addr2line doesn't output them.</div>
<div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="gmail_quote"><div class="im">
<div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="gmail_quote">
<div><br></div><div>* Symbols don't get demangled (I know, this is WIP).</div></div></blockquote><div><br></div></div><div>Yep. </div><div class="im"><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div class="gmail_quote"><div><br></div><div>* The shadow byte output is currently only really useful if you understand how ASAN works in a reasonable amount of detail. We don't provide any explanation of what they mean in the output, and even the documentation doesn't explain what the various redzone values mean. I think we can do a lot better on this particular point; I gave some sample output in my original message which shows the direction I'm thinking of.</div>
</div></blockquote><div><br></div></div><div>Yea... The main reason we output the shadow bytes is to provide some means of debugging the tool itself if something goes wrong, especially if we have a report but don't have a way to reproduce it. </div>
<div>Still, there were a few cases where the shadow bytes allowed us to understand a tricky true positive report quicker. </div><div>Additional docs/explanations may help, but my experience that users don't read long docs :( </div>
<div>So, my intention is to have less but better docs. </div><div><br></div><div>Same for other bits of output (sp, bp, stats). </div><div class="im"><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div><br></div><div>* The 'stats' output does not seem relevant in the case where ASAN traps an issue.</div><div><br></div><div>* There seems to be a tension between producing extremely detailed output (including things like bp and sp), which would be useful for a failure in a long-running process, and producing readable output (omitting such things, which are usually not interesting, and are redundant if the problem can be reproduced in a debugger). I'm not really sure what the right tradeoff is here. I certainly don't want to remove any useful information from the output.</div>
</div></blockquote><div><br></div></div><div>This tradeof is always tricky. </div><div>If you omit some info by default, you may get into a situation where you need it, but you can't reproduce the report again. </div>
<div>We've been bitten by this a lot :( </div>
<div><br></div><div>--kcc </div><div class="im"><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="gmail_quote">
<div><br></div><div>In essence, I'd like the ASAN output to be as accessible as our compile-time diagnostics. And yes, I am volunteering to help resolve the above points, if you agree that we should improve these things.</div>
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<div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="gmail_quote"><div>IOC is in some ways quite ambitious here, and provides options to not only diagnose a problem, but also to call a function which can produce a value for the operation and recover. I'm not proposing adding such a feature to Clang at this time (I'll leave that to the IOC folks).</div>
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<div><br></div><div>I would like to propose adding a runtime library for the -fcatch-undefined-behavior checks, and emit calls to relevant diagnostic functions instead of calls to @llvm.trap. I'm planning on having one library entry point per flavor of check, named __ubsan_report_<check_name>, which would be passed relevant operand values, and a pointer to a structure containing information about the context (file, line and column number, operand types as strings, and possibly a code snippet to display with the diagnostic).</div>
</blockquote><div><br></div></div><div>Do you plan to have these library functions to be marked as never-return? (if not, it will cost quite a bit of performance)</div></div></blockquote><div><br></div></div><div>Yes, that was certainly my initial plan (although John Regehr has suggested that there may be value in being able to continue after such a check failure). I would especially welcome your input into ways to emit the checks in IR, and how to design the runtime library interface, to minimize the performance burden of the checks, since I know you've done a lot of research into that.</div>
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<div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="gmail_quote"><div>Also note, that asan/tsan use the debug info to get file/line info. Do you plan to do the same, or you want to pass this info explicitly to the callback? (this will cost additional binary size and maybe some perf)</div>
</div></blockquote><div><br></div></div><div>I would like to preserve more information than can easily be extracted from the debug info (in particular, line numbers and column ranges for operands of problematic operations), so I'm intending on passing that info explicitly to the callback, but naturally if the measured impact of this is too high, I'll need to reconsider.</div>
<div><br></div><div>I think the perf impact can be mitigated by forcing the check call out of the hot path, either through just block placement, or by pushing the extra arg emission into a separate noinline function, which would be called on a check failure with a minimal set of arguments. Something like:</div>
<div><br></div><div><div>int f(int x, int y) {</div><div> return x << y;</div><div>}</div></div><div>=></div><div>linkonce_odr unnamed_addr const char __ubsan_str_8dfd1c41b32683bab10e108ac2851f2a = " return x << y;\n";</div>
<div><div>__attribute__((noreturn, noinline)) static void __ubsan_check_failure_1(int a, int b) {</div></div><div><div> static const struct LocationInfo __ubsan_locs_1[] = {</div><div><div> __FILE__, 2, 11, 12, // '<<'</div>
<div> __FILE__, 2, 9, 9, // LHS</div><div> __FILE__, 2, 14, 14 // RHS</div></div><div> };</div></div><div> __ubsan_report_left_shift_out_of_range(a, b, 32, "int", __ubsan_locs_1, str_8dfd1c41b32683bab10e108ac2851f2a);</div>
<div><div>}</div></div><div>int f(int x, int y) {</div><div> if (__builtin_expect((unsigned)y >= 32 || ((unsigned)x >> (31-y)), 0))</div><div> __ubsan_check_failure_1(x, y);</div><div><div> return x << y;</div>
<div>}</div></div></div>
</blockquote></div></div><br>
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<br></blockquote></div><br><br clear="all"><div><br></div>-- <br><div>Alexey Samsonov, MSK</div><br>