Add copy constructor and copy operator= to CScript to remove ubsan suppression #17510
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sipa
reviewed
Nov 18, 2019
| @@ -412,6 +412,9 @@ class CScript : public CScriptBase | |||
| CScript(std::vector<unsigned char>::const_iterator pbegin, std::vector<unsigned char>::const_iterator pend) : CScriptBase(pbegin, pend) { } | |||
| CScript(const unsigned char* pbegin, const unsigned char* pend) : CScriptBase(pbegin, pend) { } | |||
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| // Define a copy constructor so that operator= is a copy instead of a move to avoid undefined behavior | |||
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If this is needed, there is another problem. Why does a move cause UB? And why is a move being invoked in the first place with operator=?
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No idea. Maybe @practicalswift can shed some light here?
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For reference, one tb is this: |
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If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first. |
Agree. This is kind of scary, and working around it like this seems a bad idea if we don't understand why it happens. |
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Could this pragma in prevector.h be causing alignment issues? (doesn't it mean something like "ignore all padding requirements within the struct") |
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Smaller code to reproduce: struct Test {
uint8_t a : 8;
CScript script;
};
{
const std::array<unsigned char, 5> D{1, 2, 3, 4, 5};
Test t{0x0f, {}};
t.script = CScript{D.begin(), D.end()};
}Result: |
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Does it go away if you remove the pragma? |
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Yes, with this diff it goes away: diff --git a/src/prevector.h b/src/prevector.h
index d307495fbe..a55bcb50fe 100644
--- a/src/prevector.h
+++ b/src/prevector.h
@@ -14,7 +14,6 @@
#include <cstddef>
#include <type_traits>
-#pragma pack(push, 1)
/** Implements a drop-in replacement for std::vector<T> which stores up to N
* elements directly (without heap allocation). The types Size and Diff are
* used to store element counts, and can be any unsigned + signed type.
@@ -522,6 +521,5 @@ public:
return item_ptr(0);
}
};
-#pragma pack(pop)
#endif // BITCOIN_PREVECTOR_H |
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I would prefer that solution, then. I'm not sure in how far unaligned integers are really UB in C++ (versus architecture/implementation defined), but it'd be better to avoid that for architectures that don't support (or have slow fallback paths for) unaligned reads/writes. (E.g. SiFive U540 traps into the kernel/bootloader to handle unaligned reads and writes, with a function, in software. Wouldn't be surprised if it's 1000 times slower) |
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@laanwj John Regehr and Pascal Cuoq's excellent post "Undefined Behavior in 2017" has a nice section on Alignment Violations. Highly recommended reading! |
laanwj
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to laanwj/bitcoin
that referenced
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Nov 20, 2019
`#pragma pack(1)` prevents aligning the struct and its members to their required alignment. This can result in code that performs non-aligned reads and writes to integers and pointers, which is problematic on some architectures. It also triggers UBsan — see bitcoin#17156 (comment) and bitcoin#17510.
ajtowns
added a commit
to ajtowns/bitcoin
that referenced
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Dec 10, 2019
`#pragma pack(1)` prevents aligning the struct and its members to their required alignment. This can result in code that performs non-aligned reads and writes to integers and pointers, which is problematic on some architectures. It also triggers UBsan — see bitcoin#17156 (comment) and bitcoin#17510.
HashUnlimited
pushed a commit
to HashUnlimited/chaincoin
that referenced
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Apr 17, 2020
`#pragma pack(1)` prevents aligning the struct and its members to their required alignment. This can result in code that performs non-aligned reads and writes to integers and pointers, which is problematic on some architectures. It also triggers UBsan — see bitcoin#17156 (comment) and bitcoin#17510.
backpacker69
referenced
this pull request
in peercoin/peercoin
Mar 28, 2021
`#pragma pack(1)` prevents aligning the struct and its members to their required alignment. This can result in code that performs non-aligned reads and writes to integers and pointers, which is problematic on some architectures. It also triggers UBsan — see bitcoin/bitcoin#17156 (comment) and #17510.
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As noted in #17156 (comment), the PSBT fuzzer is triggering a UBSan condition which is fixed by this PR. Additionally, by fixing this, some UBSan suppressions can be removed.
Split from #17156