[PATCH] ident: handle NULL ai_canonname

On Fri, Sep 23, 2016 at 12:07:30AM -0400, Jeff King wrote:

Actually, it turned out to be pretty simple to reproduce (after reading 3e8a00a that John found, anyway; hooray for detailed commit messages). We just have to fake the output of gethostname(), but that is easily done since we can modify git's source. :)

So I was able to reproduce the bug, and indeed, the patch I posted fixes it. Here it is with a commit message.

Jonas, I'd be curious to know what the output of "hostname" is on your system.

-- >8 --
Subject: [PATCH] ident: handle NULL ai_canonname

We call getaddrinfo() to try to convert a short hostname into a fully-qualified one (to use it as an email domain). If there isn't a canonical name, getaddrinfo() will generally return either a NULL addrinfo list, or one in which ai->ai_canonname is a copy of the original name.

However, if the result of gethostname() looks like an IP address, then getaddrinfo() behaves differently on some systems. On OS X, it will return a "struct addrinfo" with a NULL ai_canonname, and we segfault feeding it to strchr().

This is hard to test reliably because it involves not only a system where we we have to fallback to gethostname() to come up with an ident, but also where the hostname is a number with no dots. But I was able to replicate the bug by faking a hostname, like:

    diff --git a/ident.c b/ident.c
    index e20a772..b790d28 100644
    --- a/ident.c
    +++ b/ident.c
    @@ -128,6 +128,7 @@ static void add_domainname(struct strbuf *out, int *is_bogus)
                     *is_bogus = 1;
                     return;
             }
    +        xsnprintf(buf, sizeof(buf), "1");
             if (strchr(buf, '.'))
                     strbuf_addstr(out, buf);
             else if (canonical_name(buf, out) < 0) {

and running "git var GIT_AUTHOR_IDENT" on an OS X system.

Before this patch it segfaults, and after we correctly complain of the bogus "user@1.(none)" address (though this bogus address would be suitable for non-object uses like writing reflogs).

Reported-by: Jonas Thiel <jonas.lierschied@gmx.de>
Diagnosed-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Jeff King <peff@peff.net>
---
 ident.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)