[master] a0f16286f jail_unix: Improve error reporting of failed chown()

[Nils Goroll]

commit a0f16286fa859d534eb073d9fe550de6532c0b61
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Thu Feb 13 20:57:11 2025 +0100

    jail_unix: Improve error reporting of failed chown()
    
    Before this patch, configuring an existing read only directory as the working
    directory would result in an assertion failure:
    
    $ mount | grep /tmp/ro
    swap on /tmp/ro type tmpfs (ro,relatime,inode64)
    
    $ sudo /tmp/sbin/varnishd -j unix -n /tmp/ro -a @a -b @b
    Assert error in vju_make_workdir(), mgt/mgt_jail_unix.c line 277:
      Condition((chown(dname, -1, vju_gid)) == 0) not true.
      errno = 30 (Read-only file system)
    
    This is now changed to:
    
    $ sudo /tmp/sbin/varnishd -j unix -n /tmp/ro -a @a -b @b
    Error: Cannot change group of working directory '/tmp/ro': Read-only file system
    (-? gives usage)
    
    Note: There are more {f,}chown calls in AZ(), but under the traditional model of
    a super cow powered uid 0, they should, I think, succeed once chown() of the
    working directory itself succeeded.

diff --git a/bin/varnishd/mgt/mgt_jail_unix.c b/bin/varnishd/mgt/mgt_jail_unix.c
index 9ca2f234b..18e2fcb0f 100644
--- a/bin/varnishd/mgt/mgt_jail_unix.c
+++ b/bin/varnishd/mgt/mgt_jail_unix.c
@@ -274,7 +274,11 @@ vju_make_workdir(const char *dname, const char *what, struct vsb *vsb)
 		return (1);
 	}
 	//lint -e{570}
-	AZ(chown(dname, -1, vju_gid));
+	if (chown(dname, -1, vju_gid)) {
+		MGT_Complain(C_ERR, "Cannot change group of working directory '%s': %s",
+		    dname, VAS_errtxt(errno));
+		return (1);
+	}
 	AZ(seteuid(vju_uid));
 	return (0);
 }