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fix: initialize PreTypes sentinel to fix uninitialized read on stray #endif #895

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fix: initialize PreTypes sentinel to fix uninitialized read on stray #endif #895

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14 changes: 14 additions & 0 deletions check/fixes.frm
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Original file line number Diff line number Diff line change
Expand Up @@ -5141,3 +5141,17 @@ Global F1 = E{`i' % 10}*E2*E3*E4;
.end
assert succeeded?
*--#] Issue808 :
*--#[ Issue242 :
* An #endif without a matching #if must be diagnosed deterministically.
* Previously the preprocessor peeked at the uninitialized sentinel slot of
* the PreTypes stack (PreTypes[0] with NumPreTypes == 0), so the behavior
* depended on heap garbage (Valgrind: "Conditional jump or move depends on
* uninitialised value" in DoEndif, reported at
* https://github.com/form-dev/form/issues/242).
#ifdef `A'
#message A is defined
#endif
#endif
.end
assert preprocess_error?("#endif without corresponding #if")
*--#] Issue242 :
7 changes: 7 additions & 0 deletions sources/startup.c
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Expand Up @@ -1306,6 +1306,13 @@ void StartVariables(void)
AP.MaxPreTypes = 10;
AP.NumPreTypes = 0;
AP.PreTypes = (int *)Malloc1(sizeof(int)*(AP.MaxPreTypes+1),"preprocessor types");
/*
The sentinel slot must be initialized: with an empty stack the
preprocessor termination handlers (DoEndif, DoElse, DoElseif, ...)
peek at PreTypes[NumPreTypes] == PreTypes[0], and Malloc1 does not
zero the memory.
*/
AP.PreTypes[0] = PRETYPENONE;
AP.inside.buffer = 0;
AP.inside.size = 0;

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