[Libguestfs] [libguestfs-common PATCH 2/2] options: decrypt LUKS-on-LV devices

[Laszlo Ersek]

On 02/24/22 11:43, Richard W.M. Jones wrote:
> On Wed, Feb 23, 2022 at 05:19:15PM +0100, Laszlo Ersek wrote:
>> Using the previously extracted function decrypt_mountables(), look for
>> LUKS devices on logical volumes (LVs) too.
>>
>> In the LVM-on-LUKS scheme, the names of the plaintext (decrypted) block
>> devices don't matter, as these devices host Physical Volumes, and LVM
>> enumerates PVs automatically -- there are no references to these decrypted
>> block devices in "/etc/fstab", for example. For naming such decrypted
>> devices, continue calling make_mapname().
>>
>> In the LUKS-on-LVM scheme however, the decrypted devices are supposed to
>> hold filesystems, and "/etc/fstab" may refer to them. Such decrypted
>> devices are commonly called /dev/mapper/luks-<UUID>, where <UUID> is the
>> UUID inside the LUKS header. Employ this naming when decrypting Logical
>> Volumes. Reuse make_mapname() as a fallback in this case.
>>
>> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1658126
>> Signed-off-by: Laszlo Ersek <lersek at redhat.com>
>>
>> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1658126
>> Signed-off-by: Laszlo Ersek <lersek at redhat.com>
>> ---
>>  options/decrypt.c | 24 ++++++++++++++++++------
>>  1 file changed, 18 insertions(+), 6 deletions(-)
>>
>> diff --git a/options/decrypt.c b/options/decrypt.c
>> index 9141cf5193ad..f7c1d876b3ed 100644
>> --- a/options/decrypt.c
>> +++ b/options/decrypt.c
>> @@ -38,8 +38,11 @@
>>  #include "options.h"
>>  
>>  /**
>> - * Make a LUKS map name from the partition name,
>> - * eg. C<"/dev/vda2" =E<gt> "cryptvda2">
>> + * Make a LUKS map name from the partition or logical volume name, eg.
>> + * C<"/dev/vda2" =E<gt> "cryptvda2">, or C<"/dev/vg-ssd/lv-root7" =E<gt>
>> + * "cryptvgssdlvroot7">.  Note that, in logical volume device names,
>> + * c_isalnum() eliminates the "/" separator from between the VG and the LV, so
>> + * this mapping is not unique; but for our purposes, it will do.
>>   */
>>  static void
>>  make_mapname (const char *device, char *mapname, size_t len)
>> @@ -67,7 +70,7 @@ make_mapname (const char *device, char *mapname, size_t len)
>>  
>>  static bool
>>  decrypt_mountables (guestfs_h *g, const char * const *mountables,
>> -                    struct key_store *ks)
>> +                    struct key_store *ks, bool name_decrypted_by_uuid)
>>  {
>>    bool decrypted_some = false;
>>    const char * const *mnt_scan = mountables;
>> @@ -77,7 +80,7 @@ decrypt_mountables (guestfs_h *g, const char * const *mountables,
>>      CLEANUP_FREE char *type = NULL;
>>      CLEANUP_FREE char *uuid = NULL;
>>      CLEANUP_FREE_STRING_LIST char **keys = NULL;
>> -    char mapname[32];
>> +    char mapname[512];
>>      const char * const *key_scan;
>>      const char *key;
>>  
>> @@ -102,7 +105,9 @@ decrypt_mountables (guestfs_h *g, const char * const *mountables,
>>      assert (keys[0] != NULL);
>>  
>>      /* Generate a node name for the plaintext (decrypted) device node. */
>> -    make_mapname (mountable, mapname, sizeof mapname);
>> +    if (!name_decrypted_by_uuid || uuid == NULL ||
>> +        snprintf (mapname, sizeof mapname, "luks-%s", uuid) < 0)
>> +      make_mapname (mountable, mapname, sizeof mapname);
> 
> This should all really be using asprintf, and not stack-allocating
> large arrays.  Although it's kind of not related to this change, this
> change does make the stack frame larger.

asprintf() was my first idea, but reworking make_mapname() would then
require measuring the input string lenght ;)

OK, I'll insert a patch between #1 and #2 for converting make_mapname()
to asprintf(), then I'll redo this patch with asprintf() too.

Thanks
Laszlo

> 
> Saying that, when removing gnulib (libguestfs commit 0f54df53d26), I
> dropped the warning about large stack frames:
> 
>   -dnl Warn about large stack frames, including estimates for alloca
>   -dnl and variable length arrays.
>   -gl_WARN_ADD([-Wstack-usage=10000])
> 
> I really need to add that back ...
> 
>>      /* Try each key in turn. */
>>      key_scan = (const char * const *)keys;
>> @@ -145,15 +150,22 @@ void
>>  inspect_do_decrypt (guestfs_h *g, struct key_store *ks)
>>  {
>>    CLEANUP_FREE_STRING_LIST char **partitions = guestfs_list_partitions (g);
>> +  CLEANUP_FREE_STRING_LIST char **lvs = NULL;
>>    bool need_rescan;
>>  
>>    if (partitions == NULL)
>>      exit (EXIT_FAILURE);
>>  
>> -  need_rescan = decrypt_mountables (g, (const char * const *)partitions, ks);
>> +  need_rescan = decrypt_mountables (g, (const char * const *)partitions, ks,
>> +                                    false);
>>  
>>    if (need_rescan) {
>>      if (guestfs_lvm_scan (g, 1) == -1)
>>        exit (EXIT_FAILURE);
>>    }
>> +
>> +  lvs = guestfs_lvs (g);
>> +  if (lvs == NULL)
>> +    exit (EXIT_FAILURE);
>> +  decrypt_mountables (g, (const char * const *)lvs, ks, true);
> 
> ACK
> 
> Rich.
>