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cmd/gc: avoid pointer beyond array in range loop

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This problem was discovered by reading the code.
I have not seen it in practice, nor do I have any ideas
on how to trigger it reliably in a test. But it's still worth
fixing.

TBR=ken2
CC=golang-codereviews
https://golang.org/cl/64370046
This commit is contained in:
Russ Cox 2014-02-15 20:00:57 -05:00
parent 7056b05f7a
commit 8b6ef69e23
1 changed files with 12 additions and 2 deletions
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14
src/cmd/gc/range.c
View file

@ -173,13 +173,23 @@ walkrange(Node *n)
a->list = list(list1(v1), v2);
a->rlist = list(list1(hv1), nod(OIND, hp, N));
body = list1(a);
// Advance pointer as part of increment.
// We used to advance the pointer before executing the loop body,
// but doing so would make the pointer point past the end of the
// array during the final iteration, possibly causing another unrelated
// piece of memory not to be garbage collected until the loop finished.
// Advancing during the increment ensures that the pointer p only points
// pass the end of the array during the final "p++; i++; if(i >= len(x)) break;",
// after which p is dead, so it cannot confuse the collector.
tmp = nod(OADD, hp, nodintconst(t->type->width));
tmp->type = hp->type;
tmp->typecheck = 1;
tmp->right->type = types[tptr];
tmp->right->typecheck = 1;
body = list(body, nod(OAS, hp, tmp));
a = nod(OAS, hp, tmp);
typecheck(&a, Etop);
n->nincr->ninit = list1(a);
}
break;