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Caution: clang-11.0.0 miscompiles SQLite
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Caution: clang-11.0.0 miscompiles SQLite

(1) By Richard Hipp (drh) on 2020-06-04 03:34:15 and edited on 2020-06-04 12:20:24 [history] [link] [source]

It appears that the clang-11.0.0 compiler mis-compiles sqlite3.c version 3.32.1. I have checked in a change to SQLite that appears to work around the problem. But there might be other bugs. Therefore, use clang-11.0.0 with caution and test your applications carefully!

UPDATE: I learn (from a thread on HN) that the compiler in question is pre-release.

UPDATE-2: The LLVM devs apparently saw this post on HN and quickly isolated the problem. They have a ticket. Great work, y'all!

Details:

OSSFuzz has been reporting bug 23003 against SQLite. I could not reproduce the problem on my desktop (Ubuntu with gcc-5.4.0) so I followed the OSSFuzz bug replication procedures and discovered what appears to be a problem with Clang-11.0.0 currently used by OSSFuzz.

The code that is miscompiled is lines 345-347 of the src/utf.c source file, shown below:

      c = pMem->flags;
      sqlite3VdbeMemRelease(pMem);
      pMem->flags = MEM_Str|MEM_Term|(c&(MEM_AffMask|MEM_Subtype));

From the -S output, it looks like Clang-11.0.0 is compiling these three lines as if there were written as:

      sqlite3VdbeMemRelease(pMem);
      pMem->flags = MEM_Str|MEM_Term|(pMem->flags&(MEM_AffMask|MEM_Subtype));

In other words, Clang seems to be assuming that the sqlite3VdbeMemRelease() function does not change the value of pMem->flags. But it does. My work-around is to do the bit-twiddling of pMem->flags before the function call instead of afterwards:

      c = MEM_Str|MEM_Term|(pMem->flags&(MEM_AffMask|MEM_Subtype));
      sqlite3VdbeMemRelease(pMem);
      pMem->flags = c;

Compiler Version And Build Details:

OSSFuzz reports the compiler used as:

    clang version 11.0.0 (https://github.com/llvm/llvm-project.git a6ae333a0c23fc9b0783ca45e2676abac00c6723)
    Target: x86_64-unknown-linux-gnu
    Thread model: posix

The build script compiles SQLite thusly:

clang -O1 -fno-omit-frame-pointer -gline-tables-only -DFUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION -fsanitize=address -fsanitize-address-use-after-scope -fsanitize=fuzzer-no-link -DSQLITE_MAX_LENGTH=128000000 -DSQLITE_MAX_SQL_LENGTH=128000000 -DSQLITE_MAX_MEMORY=25000000 -DSQLITE_PRINTF_PRECISION_LIMIT=1048576 -DSQLITE_DEBUG=1 -DSQLITE_MAX_PAGE_COUNT=16384 -O1 -g -I. -c -O1 -g ./sqlite3.c -o sqlite3.o