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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
399967b
C++: Do not alert on unreachable code in `cpp/incorrect-string-type-c…
jketema Jul 10, 2025
990b7f0
C++: Add change note
jketema Jul 10, 2025
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5 changes: 4 additions & 1 deletion cpp/ql/src/Security/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."
4 changes: 4 additions & 0 deletions cpp/ql/src/change-notes/2025-07-10-wchar-fp.md
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@@ -0,0 +1,4 @@
---
category: minorAnalysis
---
* The `cpp/incorrect-string-type-conversion` query no longer alerts on incorrect type conversions that occur in unreachable code.
57 changes: 44 additions & 13 deletions cpp/ql/test/query-tests/Security/CWE/CWE-704/WcharCharConversion.cpp
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Expand Up @@ -18,13 +18,13 @@ void Test()
wchar_t *lpWchar = NULL;
LPCSTR lpcstr = "b";

lpWchar = (LPWSTR)"a"; // BUG
lpWchar = (LPWSTR)lpcstr; // BUG
lpWchar = (LPWSTR)"a"; // $ Alert
lpWchar = (LPWSTR)lpcstr; // $ Alert

lpWchar = (wchar_t*)lpChar; // BUG
lpWchar = (wchar_t*)lpChar; // $ Alert

fconstWChar((LPCWSTR)lpChar); // BUG
fWChar((LPWSTR)lpChar); // BUG
fconstWChar((LPCWSTR)lpChar); // $ Alert
fWChar((LPWSTR)lpChar); // $ Alert

lpChar = (LPSTR)"a"; // Valid
lpWchar = (LPWSTR)L"a"; // Valid
Expand Down Expand Up @@ -79,33 +79,64 @@ void CheckedConversionFalsePositiveTest3(unsigned short flags, LPTSTR buffer)
if(flags & UNICODE)
lpWchar = (LPWSTR)buffer; // GOOD
else
lpWchar = (LPWSTR)buffer; // BUG
lpWchar = (LPWSTR)buffer; // $ Alert

if((flags & UNICODE) == 0x8)
lpWchar = (LPWSTR)buffer; // GOOD
else
lpWchar = (LPWSTR)buffer; // BUG
lpWchar = (LPWSTR)buffer; // $ Alert

if((flags & UNICODE) != 0x8)
lpWchar = (LPWSTR)buffer; // BUG
lpWchar = (LPWSTR)buffer; // $ Alert
else
lpWchar = (LPWSTR)buffer; // GOOD

// Bad operator precedence
if(flags & UNICODE == 0x8)
lpWchar = (LPWSTR)buffer; // BUG
lpWchar = (LPWSTR)buffer; // $ Alert
else
lpWchar = (LPWSTR)buffer; // BUG
lpWchar = (LPWSTR)buffer; // $ Alert

if((flags & UNICODE) != 0)
lpWchar = (LPWSTR)buffer; // GOOD
else
lpWchar = (LPWSTR)buffer; // BUG
lpWchar = (LPWSTR)buffer; // $ Alert

if((flags & UNICODE) == 0)
lpWchar = (LPWSTR)buffer; // BUG
lpWchar = (LPWSTR)buffer; // $ Alert
else
lpWchar = (LPWSTR)buffer; // GOOD

lpWchar = (LPWSTR)buffer; // BUG
lpWchar = (LPWSTR)buffer; // $ Alert
}

typedef unsigned long long size_t;

size_t wcslen(const wchar_t *str);
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); // GOOD -- unreachable code
}

return strlen((const char *)str);
}

template<typename C>
size_t wrong_str_len(const C *str) {
if (sizeof(C) == 1) {
return wcslen((const wchar_t *)str); // $ Alert
}

return strlen((const char *)str);
}

void test_str_len(const wchar_t *wstr, const char *str) {
size_t len =
str_len(wstr) +
str_len(str) +
wrong_str_len(wstr) +
wrong_str_len(str);
}
1 change: 1 addition & 0 deletions cpp/ql/test/query-tests/Security/CWE/CWE-704/WcharCharConversion.expected
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Expand Up @@ -11,3 +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:130:34:130:36 | str | Conversion from const char * to const wchar_t *. Use of invalid string can lead to undefined behavior. |
3 changes: 2 additions & 1 deletion cpp/ql/test/query-tests/Security/CWE/CWE-704/WcharCharConversion.qlref
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@@ -1 +1,2 @@
Security/CWE/CWE-704/WcharCharConversion.ql
query: Security/CWE/CWE-704/WcharCharConversion.ql
postprocess: utils/test/InlineExpectationsTestQuery.ql