https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=289835
Bug ID: 289835
Summary: Build failure with Clang/LLVM 22 due to alloc-size
diagnostic
Product: Base System
Version: CURRENT
Hardware: Any
OS: Any
Status: New
Severity: Affects Some People
Priority: ---
Component: kern
Assignee: [email protected]
Reporter: [email protected]
This report is to register in Bugzilla the content I posted on the mailing
list:
https://lists.freebsd.org/archives/freebsd-hackers/2025-September/005103.html
The following is a repost.
---
There is a new diagnostic, alloc-size, in clang of LLVM22 that warns if the
size given to a malloc is smaller than the size of the struct pointed to by
its destination - https://github.com/llvm/llvm-project/pull/150028
When we enable this option, in_mcast.c triggers this diagnostic, causing
the build to fail.
```
freebsd/sys/netinet/in_mcast.c:749:10: error: allocation of insufficient
size '40' for type 'struct ip_msource' with size '48' [-Werror,-Walloc-size]
749 | nims = malloc(sizeof(struct in_msource),
M_INMFILTER,
| ^
```
https://github.com/freebsd/freebsd-src/blob/stable/15/sys/netinet/in_mcast.c#L749
```
static int
imf_get_source(struct in_mfilter *imf, const struct sockaddr_in *psin,
struct in_msource **plims)
{
...
struct ip_msource *ims, *nims;
...
nims = malloc(sizeof(struct in_msource), M_INMFILTER,
M_NOWAIT | M_ZERO);
```
As the error message explained, the mismatch between struct ip_msource *
and malloc(sizeof(struct in_msource)) triggers the error.
However, when reading the source code carefully, it seems that *nims is
intentionally of type ip_msource instead of in_msource.
---
Some people might say that this is a false positive in LLVM diagnostic. But
Zhenlei's suggestion
https://lists.freebsd.org/archives/freebsd-hackers/2025-September/005106.html
could avoid the error although ideally it might be better to use a union{} for
this data as Paul mentioned in the following thread.
Additionally, the only code that actually triggers the alloc-size diagnostic is
this in_mcast and in6_mcast code. So, it would be great if the code is changed.
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