musl - Proposal to match behaviour of gethostbyname to glibc

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Message-ID: <20200313214648.2qqr5utzyqy5ewsu@wolfsden.cz>
Date: Fri, 13 Mar 2020 22:46:48 +0100
From: Wolf <wolf@...fsden.cz>
To: musl@...ts.openwall.com
Subject: Proposal to match behaviour of gethostbyname to glibc

Hello,

today I've noticed difference in behavior of gethostbyname in musl and
in glibc. Given /etc/hosts

	127.0.0.1   foo.bar foo
	127.0.0.1   bar.foo foo

and simple test program

	#include <netdb.h>
	#include <stdio.h>

	int main(int argc, char **argv) {
		struct hostent *he = gethostbyname(argv[1]);
		printf("Hostname: %s\n", he->h_name);
	}

, I've run it both under musl (alpine) and glibc (archlinux).

musl:

	/test # ./test foo
	Hostname: bar.foo

glibc:

	[root@foo test]# ./test foo
	Hostname: foo.bar

I don't think there is an actual reason to iterate through all of the
/etc/hosts and first match can be returned instead. Following patch
should in my opinion fix this.

diff --git a/src/network/lookup_name.c b/src/network/lookup_name.c
index c93263a9..da8db9d4 100644
--- a/src/network/lookup_name.c
+++ b/src/network/lookup_name.c
@@ -87,7 +87,10 @@ static int name_from_hosts(struct address buf[static MAXADDRS], char canon[stati
                for (; *p && isspace(*p); p++);
                for (z=p; *z && !isspace(*z); z++);
                *z = 0;
-               if (is_valid_hostname(p)) memcpy(canon, p, z-p+1);
+               if (is_valid_hostname(p)) {
+                       memcpy(canon, p, z-p+1);
+                       break;
+               }
        }
        __fclose_ca(f);
        return cnt ? cnt : badfam;

While this is admittedly edge case that most users will not run into, I
still think it would be nice to behave the same way as glibc does on
this one. And as a bonus, it will be *tiny* bit faster, since there
would not be any need to iterate rest of the /etc/hosts file.

Thank you for considering this,

W.

-- 
There are only two hard things in Computer Science:
cache invalidation, naming things and off-by-one errors.

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