https://bugs.exim.org/show_bug.cgi?id=1861
Bug ID: 1861
Summary: [PATCH] ubsan reports various aligned stores in sljit
Product: PCRE
Version: 8.39
Hardware: x86-64
OS: All
Status: NEW
Severity: bug
Priority: medium
Component: Code
Assignee: p...@hermes.cam.ac.uk
Reporter: marc.m...@kdab.com
CC: pcre-dev@exim.org
Created attachment 910
--> https://bugs.exim.org/attachment.cgi?id=910&action=edit
Qt patch
When running the Qt test suite under ubsan, the following errors were reported:
pcre/sljit/sljitNativeX86_64.c:342:20: runtime error: store to misaligned
address 0x000000d403c6 for type 'sljit_s32', which requires 4 byte alignment
pcre/sljit/sljitNativeX86_64.c:546:25: runtime error: store to misaligned
address 0x000000d40409 for type 'sljit_s32', which requires 4 byte alignment
pcre/sljit/sljitNativeX86_64.c:519:27: runtime error: store to misaligned
address 0x000000d40422 for type 'sljit_s32', which requires 4 byte alignment
pcre/sljit/sljitNativeX86_64.c:38:19: runtime error: store to misaligned
address 0x000000d404bb for type 'sljit_sw', which requires 8 byte alignment
pcre/sljit/sljitNativeX86_common.c:537:29: runtime error: store to
misaligned address 0x2add1f42d082 for type 'sljit_s32', which requires 4 byte
alignment
pcre/sljit/sljitNativeX86_common.c:545:29: runtime error: store to
misaligned address 0x2add1f42d23e for type 'sljit_s32', which requires 4 byte
alignment
pcre/sljit/sljitNativeX86_64.c:222:21: runtime error: store to misaligned
address 0x000000d40615 for type 'sljit_s32', which requires 4 byte alignment
pcre/sljit/sljitNativeX86_64.c:295:21: runtime error: store to misaligned
address 0x000000d4092a for type 'sljit_s32', which requires 4 byte alignment
The attached patch (taken from qtbase.git, use patch -p<n> to apply to an SVN
checkout) fixes these for the tests in the Qt test suite.
Giuseppe D'Angelo reports that it fixes all ubsan errors in the PCRE test
suite, too: http://pastebin.com/UagaK2As
Of course, we were both testing on AMD64, so the other archs presumably still
have the same problem.
From the commit message:
Fix [UBs] by providing new sljit_unaligned_store_<type> functions,
which allow for an automated conversion from the old code to
the new.
This patch fixes only those that ubsan complained about during
a run of Qt's tst_qregularexpression. The broken pattern is
used almost ubiquitously in the code, though, so these fixes
should probably be applied everywhere, too.
Background: when programming in C/C++, you are not programming
an x86 processor. The compiler is. What you're programming
against is the C/C++ abstract machine. And in that machine,
the store of an int to a misaligned address is undefined
behavior, which allows the compiler to assume it cannot happen,
and optimize accordingly. The memcpy variant should generate
the exact same code on platforms that support unaligned stores,
but, crucially, the decision is now made by the compiler,
fixing the UB.
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