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1e48ec7f6c
The C standard specifies that it's undefined behavior to dereference
NULL (even if you use & right after). The hand-rolled offsetof idiom
&(((s*)NULL)->f) is thus technically undefined. This clutters the
output of UBSan and is simple to fix: just use the real offsetof when
it's available.
Note that there's no clear statement about this point in the spec,
only several points which together converge to this:
- From N3220, 6.5.3.4:
A postfix expression followed by the -> operator and an identifier
designates a member of a structure or union object. The value is
that of the named member of the object to which the first expression
points, and is an lvalue.
- From N3220, 6.3.2.1:
An lvalue is an expression (with an object type other than void) that
potentially designates an object; if an lvalue does not designate an
object when it is evaluated, the behavior is undefined.
- From N3220, 6.5.4.4 p3:
The unary & operator yields the address of its operand. If the
operand has type "type", the result has type "pointer to type". If
the operand is the result of a unary * operator, neither that operator
nor the & operator is evaluated and the result is as if both were
omitted, except that the constraints on the operators still apply and
the result is not an lvalue. Similarly, if the operand is the result
of a [] operator, neither the & operator nor the unary * that is
implied by the [] is evaluated and the result is as if the & operator
were removed and the [] operator were changed to a + operator.
=> In short, this is saying that C guarantees these identities:
1. &(*p) is equivalent to p
2. &(p[n]) is equivalent to p + n
As a consequence, &(*p) doesn't result in the evaluation of *p, only
the evaluation of p (and similar for []). There is no corresponding
special carve-out for ->.
See also: https://pvs-studio.com/en/blog/posts/cpp/0306/
After this patch, HAProxy can run without crashing after building w/
clang-19 -fsanitize=undefined -fno-sanitize=function,alignment
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|---|---|---|
| .. | ||
| atomic-ops.h | ||
| cebtree-prv.h | ||
| cebtree.h | ||
| cebu32_tree.h | ||
| cebu64_tree.h | ||
| cebua_tree.h | ||
| cebub_tree.h | ||
| cebuib_tree.h | ||
| cebuis_tree.h | ||
| cebul_tree.h | ||
| cebus_tree.h | ||
| eb32sctree.h | ||
| eb32tree.h | ||
| eb64tree.h | ||
| ebimtree.h | ||
| ebistree.h | ||
| ebmbtree.h | ||
| ebpttree.h | ||
| ebsttree.h | ||
| ebtree-t.h | ||
| ebtree.h | ||
| ist.h | ||
| lru.h | ||
| mjson.h | ||
| mt_list.h | ||
| plock.h | ||
| sha1.h | ||
| slz-tables.h | ||
| slz.h | ||
| xxhash.h | ||