redirect: Add new override on getpwuid

This is required for some Aspyr ports which will form their own fixed
paths to directories based on getpwuid() as opposed to using the environment
`HOME` variable.

With this we force the home directory to be consistent under snapd and
alleviate any issues with denials based on home directory lookups.

Signed-off-by: Ikey Doherty <ikey@solus-project.com>

src/redirect/main.c

@@ -19,6 +19,13 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "config.h"
+
+/* Expose getpwuid override in snapd */
+#ifdef HAVE_SNAPD_SUPPORT
+#include <pwd.h>
+#endif
+
 #include "../common/common.h"
 #include "../common/files.h"
 #include "../common/log.h"
@@ -82,6 +89,9 @@ static LsiRedirectProfile *lsi_profile = NULL;
 static LsiSymbolBinding lsi_libc_bindings[] = {
         SYMBOL_BINDING(libc, open),
         SYMBOL_BINDING(libc, fopen64),
+#ifdef HAVE_SNAPD_SUPPORT
+        SYMBOL_BINDING(libc, getpwuid),
+#endif
 };
 
 /**
@@ -335,6 +345,32 @@ fallback_open:
         return lsi_table.fopen64(p, modes);
 }
 
+#ifdef HAVE_SNAPD_SUPPORT
+
+#include <unistd.h>
+
+_nica_public_ struct passwd *getpwuid(uid_t uid)
+{
+        /* Must ensure we're **really** initialised, as we might see open happen
+         * before the constructor..
+         */
+        lsi_redirect_init_tables();
+
+        struct passwd *ret = NULL;
+        const char *snap_root = getenv("SNAP_USER_COMMON");
+
+        /* If they're requesting our uid and SNAP_USER_COMMON is set, then
+         * let us override the home directory to be correct.
+         */
+        ret = lsi_table.getpwuid(uid);
+        if (uid == getuid() && snap_root && *snap_root) {
+                ret->pw_dir = (char *)snap_root;
+        }
+        return ret;
+}
+
+#endif
+
 /*
  * Editor modelines  -  https://www.wireshark.org/tools/modelines.html
  *

src/redirect/private.h

@@ -14,6 +14,12 @@
 #include <stdbool.h>
 #include <stdio.h>
 
+#include "config.h"
+
+#ifdef HAVE_SNAPD_SUPPORT
+#include <pwd.h>
+#endif
+
 /**
  * Handle definitions
  */
@@ -21,6 +27,10 @@ typedef int (*lsi_open_file)(const char *p, int flags, mode_t mode);
 
 typedef FILE *(*lsi_fopen64_file)(const char *p, const char *modes);
 
+#ifdef HAVE_SNAPD_SUPPORT
+typedef struct passwd *(*lsi_getpwuid)(uid_t uid);
+#endif
+
 /**
  * Global storage of handles for nicer organisation.
  */
@@ -28,6 +38,10 @@ typedef struct LsiRedirectTable {
         lsi_open_file open;
         lsi_fopen64_file fopen64;
 
+#ifdef HAVE_SNAPD_SUPPORT
+        lsi_getpwuid getpwuid;
+#endif
+
         /* Allow future handle opens.. */
         struct {
                 void *libc;

src/redirect/sym.map

@@ -1,6 +1,7 @@
 {
   global:
     fopen64;
+    getpwuid;
     open;
   local:
     *;