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wasm2c: avoid indexing null pointer triggering undefined behavior

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Using zig cc to compile and run wasm2c on zig.wasm on Windows triggers
what appears to be a sanitizer crash.  The FuncGen reuse array pointer is
initialized to null and at some point it's resized to a length of zero,
which triggers this code to execute:

    memcpy(&self->reuse[self->reuse_i], &self->reuse[reuse_top], sizeof(uint32_t) * reuse_n);

Given the current values, this equates to:

    memcpy(&(NULL)[0], &(NULL)[0], 0);

Taking the address of the first element of a null pointer doesn't trigger
any actual runtime problem, since the pointer won't be dereferenced because
were passing 0 as the length to memcpy, however, it seems that the C spec
considers indexing a null pointer to be undefined behavior even if you
don't use the resulting value (or are just taking the address of an
indexed pointer).
This commit is contained in:
Jonathan Marler 2024-06-16 07:45:30 -06:00 • committed by Jacob Young
parent 455899668b
commit 1165e13c25
1 changed files with 4 additions and 2 deletions
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6
stage1/FuncGen.h
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@ -179,8 +179,10 @@ static void FuncGen_blockBegin(struct FuncGen *self, FILE *out, enum WasmOpcode
self->reuse = realloc(self->reuse, sizeof(uint32_t) * self->reuse_len);
if (self->reuse == NULL) panic("out of memory");
}
memcpy(&self->reuse[self->reuse_i], &self->reuse[reuse_top], sizeof(uint32_t) * reuse_n);
self->reuse_i += reuse_n;
if (reuse_n != 0) {
memcpy(&self->reuse[self->reuse_i], &self->reuse[reuse_top], sizeof(uint32_t) * reuse_n);
self->reuse_i += reuse_n;
}
}
static enum WasmOpcode FuncGen_blockKind(const struct FuncGen *self, uint32_t label_idx) {