fs_mgr: adding fs_mgr_get_slot_suffix() public API

The function returns "_a" or "_b" based on two possible values in
kernel cmdline:

  - androidboot.slot = a or b OR
  - androidboot.slot_suffix = _a or _b

Bug: 33254008
Bug: 36533366
Test: boot sailfish
Change-Id: Ia0a524e4145ebf61af5821f42ecad212c95ed748

fs_mgr/fs_mgr_avb.cpp

@@ -489,15 +489,9 @@ int fs_mgr_load_vbmeta_images(struct fstab* fstab) {
     // Sets requested_partitions to nullptr as it's to copy the contents
     // of HASH partitions into fs_mgr_avb_verify_data, which is not required as
     // fs_mgr only deals with HASHTREE partitions.
-    const char *requested_partitions[] = {nullptr};
-    std::string ab_suffix;
-    std::string slot;
-    if (fs_mgr_get_boot_config("slot", &slot)) {
-        ab_suffix = "_" + slot;
-    } else {
-        // remove slot_suffix once bootloaders update to new androidboot.slot param
-        fs_mgr_get_boot_config("slot_suffix", &ab_suffix);
-    }
+    const char* requested_partitions[] = {nullptr};
+    std::string ab_suffix = fs_mgr_get_slot_suffix();
+
     AvbSlotVerifyResult verify_result =
         avb_slot_verify(fs_mgr_avb_ops, requested_partitions, ab_suffix.c_str(),
                         fs_mgr_vbmeta_prop.allow_verification_error, &fs_mgr_avb_verify_data);

fs_mgr/fs_mgr_fstab.cpp

@@ -526,7 +526,7 @@ static struct fstab *fs_mgr_read_fstab_file(FILE *fstab_file)
         cnt++;
     }
     /* If an A/B partition, modify block device to be the real block device */
-    if (fs_mgr_update_for_slotselect(fstab) != 0) {
+    if (!fs_mgr_update_for_slotselect(fstab)) {
         LERROR << "Error updating for slotselect";
         goto err;
     }

fs_mgr/fs_mgr_priv.h

@@ -41,8 +41,6 @@
 #define PWARNING PLOG(WARNING) << FS_MGR_TAG
 #define PERROR   PLOG(ERROR) << FS_MGR_TAG
 
-__BEGIN_DECLS
-
 #define CRYPTO_TMPFS_OPTIONS "size=256m,mode=0771,uid=1000,gid=1000"
 
 #define WAIT_TIMEOUT 20
@@ -114,10 +112,8 @@ __BEGIN_DECLS
 
 int fs_mgr_set_blk_ro(const char *blockdev);
 int fs_mgr_test_access(const char *device);
-int fs_mgr_update_for_slotselect(struct fstab *fstab);
+bool fs_mgr_update_for_slotselect(struct fstab *fstab);
 bool is_dt_compatible();
 bool is_device_secure();
 
-__END_DECLS
-
 #endif /* __CORE_FS_MGR_PRIV_H */

fs_mgr/fs_mgr_slotselect.cpp

@@ -16,37 +16,47 @@
 
 #include <stdio.h>
 
+#include <string>
+
 #include "fs_mgr.h"
 #include "fs_mgr_priv.h"
 
-// Updates |fstab| for slot_suffix. Returns 0 on success, -1 on error.
-int fs_mgr_update_for_slotselect(struct fstab *fstab)
-{
+// Returns "_a" or "_b" based on two possible values in kernel cmdline:
+//   - androidboot.slot = a or b OR
+//   - androidboot.slot_suffix = _a or _b
+// TODO: remove slot_suffix once it's deprecated.
+std::string fs_mgr_get_slot_suffix() {
+    std::string slot;
+    std::string ab_suffix;
+
+    if (fs_mgr_get_boot_config("slot", &slot)) {
+        ab_suffix = "_" + slot;
+    } else if (!fs_mgr_get_boot_config("slot_suffix", &ab_suffix)) {
+        ab_suffix = "";
+    }
+    return ab_suffix;
+}
+
+// Updates |fstab| for slot_suffix. Returns true on success, false on error.
+bool fs_mgr_update_for_slotselect(struct fstab *fstab) {
     int n;
-    int got_suffix = 0;
-    std::string suffix;
+    std::string ab_suffix;
 
     for (n = 0; n < fstab->num_entries; n++) {
         if (fstab->recs[n].fs_mgr_flags & MF_SLOTSELECT) {
             char *tmp;
-
-            if (!got_suffix) {
-                std::string slot;
-                if (fs_mgr_get_boot_config("slot", &slot)) {
-                    suffix = "_" + slot;
-                } else if (!fs_mgr_get_boot_config("slot_suffix", &suffix)) {
-                    // remove slot_suffix once bootloaders update to new androidboot.slot param
-                    return -1;
-                }
+            if (ab_suffix.empty()) {
+                ab_suffix = fs_mgr_get_slot_suffix();
+                // Returns false as non A/B devices should not have MF_SLOTSELECT.
+                if (ab_suffix.empty()) return false;
             }
-
-            if (asprintf(&tmp, "%s%s", fstab->recs[n].blk_device, suffix.c_str()) > 0) {
+            if (asprintf(&tmp, "%s%s", fstab->recs[n].blk_device, ab_suffix.c_str()) > 0) {
                 free(fstab->recs[n].blk_device);
                 fstab->recs[n].blk_device = tmp;
             } else {
-                return -1;
+                return false;
             }
         }
     }
-    return 0;
+    return true;
 }

fs_mgr/include/fs_mgr.h

@@ -22,6 +22,12 @@
 #include <stdbool.h>
 #include <linux/dm-ioctl.h>
 
+// C++ only headers
+// TODO: move this into separate header files under include/fs_mgr/*.h
+#ifdef __cplusplus
+#include <string>
+#endif
+
 // Magic number at start of verity metadata
 #define VERITY_METADATA_MAGIC_NUMBER 0xb001b001
 
@@ -29,9 +35,7 @@
 // turn verity off in userdebug builds.
 #define VERITY_METADATA_MAGIC_DISABLE 0x46464f56 // "VOFF"
 
-#ifdef __cplusplus
-extern "C" {
-#endif
+__BEGIN_DECLS
 
 // Verity modes
 enum verity_mode {
@@ -139,8 +143,12 @@ int fs_mgr_do_format(struct fstab_rec *fstab, bool reserve_footer);
 #define FS_MGR_SETUP_VERITY_SUCCESS 0
 int fs_mgr_setup_verity(struct fstab_rec *fstab, bool wait_for_verity_dev);
 
+__END_DECLS
+
+// C++ only functions
+// TODO: move this into separate header files under include/fs_mgr/*.h
 #ifdef __cplusplus
-}
+std::string fs_mgr_get_slot_suffix();
 #endif
 
 #endif /* __CORE_FS_MGR_H */