On Wed, Dec 25, 2019 at 04:34:11PM -0800, David Miller wrote:

From: "A. Wilcox" <redacted>
Date: Sun, 22 Dec 2019 00:02:27 -0600

quoted

@@ -83,6 +83,13 @@
 
 #define IPV4_BEET_PHMAXLEN 8
 
+/* Allow libcs to deactivate this - musl has its own copy in <netinet/ip.h> */
+
+#ifndef __UAPI_DEF_IPHDR
+#define __UAPI_DEF_IPHDR	1
+#endif

How is this a musl-only problem?

I don't think it is, unless glibc's includes linux/ip.h to get the
definition, which does not seem to be the case -- at least not on the
Debian system I had handy to check on.

I see that glibc also defines struct iphdr
in netinet/ip.h, so why doesn't it also suffer from this?

Maybe it does.

I find it really strange that this, therefore, only happens for musl
and we haven't had thousands of reports of this conflict with glibc
over the years.

It's possible that there's software that's including just one of the
headers conditional on __GLIBC__, and including both otherwise, or
something like that. Arguably this should be considered unsupported
usage; there are plenty of headers where that doesn't work and
shouldn't be expected to.

I want an explanation, and suitably appropriate adjustments to the commit
message and comments of this change.

Agreed. Commit messages should not imply that something is a
musl-specific workaround when it's generally the right thing to do.

Rich