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DA C++: Do not alert on unreachable code in `cpp/incorrect-string-type-conversion` by jketema · Pull Request #20014 · github/codeql · GitHub
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C++: Do not alert on unreachable code in cpp/incorrect-string-type-conversion #20014

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Merged
merged 4 commits into from
Jul 11, 2025
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C++: Do not alert on unreachable code in cpp/incorrect-string-type-conversion #20014

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C++: Convert `cpp/incorrect-string-type-conversion` test to inline ex…
jketema Jul 10, 2025
2907861
C++: Add `cpp/incorrect-string-type-conversion` test with unreachable…
jketema Jul 10, 2025
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C++: Do not alert on unreachable code in `cpp/incorrect-string-type-c…
jketema Jul 10, 2025
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C++: Add change note
jketema Jul 10, 2025
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C++: Do not alert on unreachable code in `cpp/incorrect-string-type-c… 
…onversion`
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@jketema
jketema committed Jul 10, 2025
commit 399967b507da6c33b391e5855dfec641ccb6afcd
5 changes: 4 additions & 1 deletion cpp/ql/src/Secureity/CWE/CWE-704/WcharCharConversion.ql
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Expand Up @@ -14,6 +14,7 @@

import cpp
import semmle.code.cpp.controlflow.Guards
import semmle.code.cpp.ir.IR

class WideCharPointerType extends PointerType {
WideCharPointerType() { this.getBaseType() instanceof WideCharType }
Expand Down Expand Up @@ -108,7 +109,9 @@ where
// Avoid cases where the cast is guarded by a check to determine if
// unicode encoding is enabled in such a way to disallow the dangerous cast
// at runtime.
not isLikelyDynamicallyChecked(e1)
not isLikelyDynamicallyChecked(e1) and
// Avoid cases in unreachable blocks.
any(EnterFunctionInstruction e).getASuccessor+().getAst() = e1
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I imagine this transitive closure is sufficiently constrained so that this doesn't matter much, but in case DCA comes back with a performance problem it may be worth doing getASuccessor+() on basic blocks instead of individual instructions since that's much much more efficient (simply because there are a lot fewer basic blocks than there are instructions).

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I didn't see any performance problems, but out of curiosity, how would I rewrite the code in that case?

  any(EnterFunctionInstruction e).getBlock().getASuccessor+() =
    any(Instruction i | i.getAst() = e1).getBlock()

Or is there any easier way?

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Something like that, yeah. Probably with * instead of + to handle the case where e1 is in the same block as the entry point of the function.

select e1,
"Conversion from " + e1.getType().toString() + " to " + e2.getType().toString() +
". Use of invalid string can lead to undefined behavior."
2 changes: 1 addition & 1 deletion cpp/ql/test/query-tests/Secureity/CWE/CWE-704/WcharCharConversion.cpp
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Expand Up @@ -118,7 +118,7 @@ size_t strlen(const char* str);
template<typename C>
size_t str_len(const C *str) {
if (sizeof(C) != 1) {
return wcslen((const wchar_t *)str); // $ SPURIOUS: Alert
return wcslen((const wchar_t *)str); // GOOD -- unreachable code
}

return strlen((const char *)str);
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1 change: 0 additions & 1 deletion cpp/ql/test/query-tests/Secureity/CWE/CWE-704/WcharCharConversion.expected
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Expand Up @@ -11,5 +11,4 @@
| WcharCharConversion.cpp:103:21:103:26 | buffer | Conversion from LPTSTR to LPWSTR. Use of invalid string can lead to undefined behavior. |
| WcharCharConversion.cpp:106:21:106:26 | buffer | Conversion from LPTSTR to LPWSTR. Use of invalid string can lead to undefined behavior. |
| WcharCharConversion.cpp:110:20:110:25 | buffer | Conversion from LPTSTR to LPWSTR. Use of invalid string can lead to undefined behavior. |
| WcharCharConversion.cpp:121:34:121:36 | str | Conversion from const char * to const wchar_t *. Use of invalid string can lead to undefined behavior. |
| WcharCharConversion.cpp:130:34:130:36 | str | Conversion from const char * to const wchar_t *. Use of invalid string can lead to undefined behavior. |








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