environment: Add --user.

This change allows overriding the home directory of all filesystem mappings to
help hide the identity of the calling user in a container.

* doc/guix.texi (Invoking guix environment)[--container]: Mention --user.
[--user]: Add item.
* guix/scripts/environment.scm (show-help): Add --user.
(%options): Add --user.
(launch-environment/container) Add 'user' parameter.  Update doc.  Override
'user-mappings' using 'override-user-mappings'.  Consider override for chdir.
(mock-passwd, user-override-home, overrid-euser-dir): New procedures.
(guix-environment): Disallow --user without --container.  Provide user to
'launch-environment/container'.
* tests/guix-environment.sh: Add user test.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -7286,10 +7286,11 @@ Attempt to build for @var{system}---e.g., @code{i686-linux}.
 @cindex container
 Run @var{command} within an isolated container.  The current working
 directory outside the container is mapped inside the container.
-Additionally, a dummy home directory is created that matches the current
-user's home directory, and @file{/etc/passwd} is configured accordingly.
-The spawned process runs as the current user outside the container, but
-has root privileges in the context of the container.
+Additionally, unless overridden with @code{--user}, a dummy home
+directory is created that matches the current user's home directory, and
+@file{/etc/passwd} is configured accordingly.  The spawned process runs
+as the current user outside the container, but has root privileges in
+the context of the container.
 
 @item --network
 @itemx -N
@@ -7313,6 +7314,31 @@ example, the @code{fontconfig} package inspects
 @code{--link-profile} allows these programs to behave as expected within
 the environment.
 
+@item --user=@var{user}
+@itemx -u @var{user}
+For containers, use the username @var{user} in place of the current
+user.  The generated @file{/etc/passwd} entry within the container will
+contain the name @var{user}; the home directory will be
+@file{/home/USER}; and no user GECOS data will be copied.  @var{user}
+need not exist on the system.
+
+Additionally, any shared or exposed path (see @code{--share} and
+@code{--expose} respectively) whose target is within the current user's
+home directory will be remapped relative to @file{/home/USER}; this
+includes the automatic mapping of the current working directory.
+
+@example
+# will expose paths as /home/foo/wd, /home/foo/test, and /home/foo/target
+cd $HOME/wd
+guix environment --container --user=foo \
+     --expose=$HOME/test \
+     --expose=/tmp/target=$HOME/target
+@end example
+
+While this will limit the leaking of user identity through home paths
+and each of the user fields, this is only one useful component of a
+broader privacy/anonymity solution---not one in and of itself.
+
 @item --expose=@var{source}[=@var{target}]
 For containers, expose the file system @var{source} from the host system
 as the read-only file system @var{target} within the container.  If

guix/scripts/environment.scm

@@ -163,6 +163,10 @@ (define (show-help)
   (display (G_ "
   -P, --link-profile     link environment profile to ~/.guix-profile within
                          an isolated container"))
+  (display (G_ "
+  -u, --user=USER        instead of copying the name and home of the current
+                         user into an isolated container, use the name USER
+                         with home directory /home/USER"))
   (display (G_ "
       --share=SPEC       for containers, share writable host file system
                          according to SPEC"))
@@ -250,6 +254,10 @@ (define %options
          (option '(#\P "link-profile") #f #f
                  (lambda (opt name arg result)
                    (alist-cons 'link-profile? #t result)))
+         (option '(#\u "user") #t #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'user arg
+                               (alist-delete 'user result eq?))))
          (option '("share") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'file-system-mapping
@@ -410,43 +418,50 @@ (define (launch-environment/fork command inputs paths pure?)
     (pid (match (waitpid pid)
            ((_ . status) status)))))
 
-(define* (launch-environment/container #:key command bash user-mappings
+(define* (launch-environment/container #:key command bash user user-mappings
                                        profile paths link-profile? network?)
   "Run COMMAND within a container that features the software in PROFILE.
 Environment variables are set according to PATHS, a list of native search
 paths.  The global shell is BASH, a file name for a GNU Bash binary in the
 store.  When NETWORK?, access to the host system network is permitted.
 USER-MAPPINGS, a list of file system mappings, contains the user-specified
-host file systems to mount inside the container.  LINK-PROFILE? creates a
-symbolic link from ~/.guix-profile to the environment profile."
+host file systems to mount inside the container.  If USER is not #f, each
+target of USER-MAPPINGS will be re-written relative to '/home/USER', and USER
+will be used for the passwd entry.  LINK-PROFILE? creates a symbolic link from
+~/.guix-profile to the environment profile."
   (mlet %store-monad ((reqs (inputs->requisites
                              (list (direct-store-path bash) profile))))
     (return
      (let* ((cwd      (getcwd))
-            (passwd   (getpwuid (getuid)))
+            (home     (getenv "HOME"))
+            (passwd   (mock-passwd (getpwuid (getuid))
+                                   user
+                                   bash))
             (home-dir (passwd:dir passwd))
             ;; Bind-mount all requisite store items, user-specified mappings,
             ;; /bin/sh, the current working directory, and possibly networking
             ;; configuration files within the container.
             (mappings
-             (append user-mappings
-                     ;; Current working directory.
-                     (list (file-system-mapping
-                            (source cwd)
-                            (target cwd)
-                            (writable? #t)))
-                     ;; When in Rome, do as Nix build.cc does: Automagically
-                     ;; map common network configuration files.
-                     (if network?
-                         %network-file-mappings
-                         '())
-                     ;; Mappings for the union closure of all inputs.
-                     (map (lambda (dir)
-                            (file-system-mapping
-                             (source dir)
-                             (target dir)
-                             (writable? #f)))
-                          reqs)))
+             (override-user-mappings
+              user home
+              (append user-mappings
+                      ;; Current working directory.
+                      (list (file-system-mapping
+                             (source cwd)
+                             (target cwd)
+                             (writable? #t)))
+                      ;; When in Rome, do as Nix build.cc does: Automagically
+                      ;; map common network configuration files.
+                      (if network?
+                          %network-file-mappings
+                          '())
+                      ;; Mappings for the union closure of all inputs.
+                      (map (lambda (dir)
+                             (file-system-mapping
+                              (source dir)
+                              (target dir)
+                              (writable? #f)))
+                           reqs))))
             (file-systems (append %container-file-systems
                                   (map file-system-mapping->bind-mount
                                        mappings))))
@@ -467,8 +482,7 @@ (define* (launch-environment/container #:key command bash user-mappings
                       ;; The same variables as in Nix's 'build.cc'.
                       '("TMPDIR" "TEMPDIR" "TMP" "TEMP"))
 
-            ;; Create a dummy home directory under the same name as on the
-            ;; host.
+            ;; Create a dummy home directory.
             (mkdir-p home-dir)
             (setenv "HOME" home-dir)
 
@@ -495,7 +509,7 @@ (define* (launch-environment/container #:key command bash user-mappings
 
             ;; For convenience, start in the user's current working
             ;; directory rather than the root directory.
-            (chdir cwd)
+            (chdir (override-user-dir user home cwd))
 
             (primitive-exit/status
              ;; A container's environment is already purified, so no need to
@@ -505,6 +519,60 @@ (define* (launch-environment/container #:key command bash user-mappings
                            (delq 'net %namespaces) ; share host network
                            %namespaces)))))))
 
+(define (mock-passwd passwd user-override shell)
+  "Generate mock information for '/etc/passwd'.  If USER-OVERRIDE is not '#f',
+it is expected to be a string representing the mock username; it will produce
+a user of that name, with a home directory of '/home/USER-OVERRIDE', and no
+GECOS field.  If USER-OVERRIDE is '#f', data will be inherited from PASSWD.
+In either case, the shadow password and UID/GID are cleared, since the user
+runs as root within the container.  SHELL will always be used in place of the
+shell in PASSWD.
+
+The resulting vector is suitable for use with Guile's POSIX user procedures.
+
+See passwd(5) for more information each of the fields."
+  (if user-override
+      (vector
+       user-override
+        "x" "0" "0"  ;; no shadow, user is now root
+        ""           ;; no personal information
+        (user-override-home user-override)
+        shell)
+      (vector
+       (passwd:name passwd)
+        "x" "0" "0"  ;; no shadow, user is now root
+        (passwd:gecos passwd)
+        (passwd:dir passwd)
+        shell)))
+
+(define (user-override-home user)
+  "Return home directory for override user USER."
+  (string-append "/home/" user))
+
+(define (override-user-mappings user home mappings)
+  "If a username USER is provided, rewrite each HOME prefix in file system
+mappings MAPPINGS to a home directory determined by 'override-user-dir';
+otherwise, return MAPPINGS."
+  (if (not user)
+      mappings
+      (map (lambda (mapping)
+             (let ((target (file-system-mapping-target mapping)))
+               (if (string-prefix? home target)
+                   (file-system-mapping
+                    (source    (file-system-mapping-source mapping))
+                    (target    (override-user-dir user home target))
+                    (writable? (file-system-mapping-writable? mapping)))
+                   mapping)))
+           mappings)))
+
+(define (override-user-dir user home dir)
+  "If username USER is provided, overwrite string prefix HOME in DIR with a
+directory determined by 'user-override-home'; otherwise, return DIR."
+  (if (and user (string-prefix? home dir))
+      (string-append (user-override-home user)
+                     (substring dir (string-length home)))
+      dir))
+
 (define (link-environment profile home-dir)
   "Create a symbolic link from HOME-DIR/.guix-profile to PROFILE."
   (let ((profile-dir (string-append home-dir "/.guix-profile")))
@@ -592,6 +660,7 @@ (define (guix-environment . args)
            (container? (assoc-ref opts 'container?))
            (link-prof? (assoc-ref opts 'link-profile?))
            (network?   (assoc-ref opts 'network?))
+           (user       (assoc-ref opts 'user))
            (bootstrap? (assoc-ref opts 'bootstrap?))
            (system     (assoc-ref opts 'system))
            (command    (or (assoc-ref opts 'exec)
@@ -626,6 +695,8 @@ (define (guix-environment . args)
 
       (when (and (not container?) link-prof?)
         (leave (G_ "'--link-profile' cannot be used without '--container'~%")))
+      (when (and (not container?) user)
+        (leave (G_ "'--user' cannot be used without '--container'~%")))
 
       (with-store store
         (set-build-options-from-command-line store opts)
@@ -673,6 +744,7 @@ (define (guix-environment . args)
                                             "/bin/sh"))))
                     (launch-environment/container #:command command
                                                   #:bash bash-binary
+                                                  #:user user
                                                   #:user-mappings mappings
                                                   #:profile profile
                                                   #:paths paths

tests/guix-environment-container.sh

@@ -109,6 +109,17 @@ rm $tmpdir/mounts
              -- guile -c "$linktest"
 )
 
+# Test that user can be mocked.
+usertest='(exit (and (string=? (getenv "HOME") "/home/foognu")
+                     (string=? (passwd:name (getpwuid 0)) "foognu")
+                     (file-exists? "/home/foognu/umock")))'
+touch "$tmpdir/umock"
+HOME="$tmpdir" guix environment --bootstrap --container --user=foognu \
+     --ad-hoc guile-bootstrap --pure \
+     --share="$tmpdir/umock" \
+     -- guile -c "$usertest"
+
+
 # Check the exit code.
 
 abnormal_exit_code="