[llvm-bugs] [Bug 35485] New: clang is not resilient to source files changing length underneath it

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35485

            Bug ID: 35485
           Summary: clang is not resilient to source files changing length
                    underneath it
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: caustin at gmail.com
                CC: llvm-bugs at lists.llvm.org

At Facebook, engineers have reported that clang will sometimes segfault if the
developer saves a file in the middle of compilation.

While tracking down an unrelated bug, I noticed that this appears to be because
SourceManager has UserFilesAreVolatile set to false which eventually causes
MemoryBuffer::getOpenFile to be called with isVolatile false, which memory maps
it.  When memory mapping a file whose length has changed underneath it,
accessing pages beyond the end of the backing file results in a SIGBUS.  In
addition, I believe line number computation expects a trailing nul byte, which
is implemented by mapping one byte larger than the file (assuming the file is
not exactly on a page boundary).  But if the file has since expanded, that byte
is no longer null, and clang SIGSEGVs while computing line numbers.

It seems like UserFilesAreVolatile should be set true for non-system files.

#35333 looks related. And possibly #20880.

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