[llvm-bugs] [Bug 27627] New: [ASan] False positive stack-buffer-underflow with mmap and boost::context

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27627

            Bug ID: 27627
           Summary: [ASan] False positive stack-buffer-underflow with mmap
                    and boost::context
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Miscellaneous Instrumentation passes
          Assignee: unassignedbugs at nondot.org
          Reporter: mhx at fb.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 16300
  --> https://llvm.org/bugs/attachment.cgi?id=16300&action=edit
Repro for the described bug

For a couple of weeks we've been seeing errors in our ASan-enabled builds that
upon closer investigation appear to be false positives. I've spent some time
trying to reduce the code to the attached repro, which only depends on the
boost context library.

In a nutshell, the error triggers when:

1. A stack frame for boost::context is allocated via mmap

2. An actual context switch happens to a function that uses the stack

3. The region is munmap'd after switching back to the main context

4. We mmap a new region that happens to overlap with the previously freed one
(in the repro code, this overlap is forced, in the original code it happens
randomly)

5. Now, reading from memory inside that region that is close to the stack frame
used before for the context switch will trigger the stack-buffer-underflow
error

The attached repro contains some more details and includes a command line to
trigger the FP ASan error. The problem can be reproduced with any recent
version of clang (or gcc).

Happy to provide more details if needed!

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