Re: [PATCH] Add grub-fstest cat subcommand

[Vladimir 'φ-coder/phcoder' Serbinenko]

On 21.03.2011 19:17, Colin Watson wrote:
> This would make it easier to use grub-fstest to inspect files in
> filesystems supported by GRUB without needing to mount the device.  The
> same thing can be done using the cp subcommand, but that requires a
> temporary file which often isn't needed.  A cat subcommand is generally
> more convenient.
>   
Patch is ok after 1.99.
> For context: temporarily mounting filesystems in the context of an OS
> installer or a tool such as os-prober can be problematic on Linux.  In
> particular, if you want to inspect files on a journalled filesystem,
> then you have three choices to the best of my knowledge:
>
>   1) Just go ahead and mount it read-only.  This seems superficially
>      sensible.  Unfortunately, Linux's journalling filesystem
>      implementations will write to the block device anyway.  If the
>      filesystem was hibernated, disaster will ensue.  (And, on
>      principle, the kinds of places where I'm doing this kind of
>      read-only mount, such as os-prober, should never write to the
>      disk.)
>
>   2) Mark the block device read-only using blockdev, to ensure that even
>      journalling filesystems will never write to it.  This solves the
>      hibernation problem, but has two other flaws.  If the journal needs
>      recovery, mounting will fail when an ordinary read-only mount would
>      succeed; and blockdev has system-wide effect, so this will
>      interfere with anything unlucky enough to be trying to mount the
>      filesystem for real at the same time.
>
>   3) Create a device-mapper snapshot of the block device and operate on
>      that.  Unfortunately, if the block device is not itself a
>      device-mapper device, this increments the reference count of the
>      block device such that you can't mount the filesystem on it
>      ("/dev/foo already mounted or /mnt busy").
>
> I'm thus getting increasingly tempted to use grub-fstest for this kind
> of application wherever possible.  Of course we need a fallback for
> architectures where GRUB doesn't yet work, but aside from that I suspect
> that it would make things significantly more reliable.
>
> 2011-03-21  Colin Watson  <address@hidden>
>
>       * util/grub-fstest.c (cmd_cat): New function.
>       (fstest): Handle CMD_CAT.
>       (options): Add cat.
>       (argp_parser): Handle cat.
>
> === modified file 'util/grub-fstest.c'
> --- util/grub-fstest.c        2010-09-20 18:09:31 +0000
> +++ util/grub-fstest.c        2011-03-21 17:32:32 +0000
> @@ -54,12 +54,15 @@ execute_command (char *name, int n, char
>    return (cmd->func) (cmd, n, args);
>  }
>  
> -#define CMD_LS          1
> -#define CMD_CP          2
> -#define CMD_CMP         3
> -#define CMD_HEX         4
> -#define CMD_CRC         6
> -#define CMD_BLOCKLIST   7
> +enum {
> +  CMD_LS = 1,
> +  CMD_CP,
> +  CMD_CAT,
> +  CMD_CMP,
> +  CMD_HEX,
> +  CMD_CRC,
> +  CMD_BLOCKLIST
> +};
>  
>  #define BUF_SIZE  32256
>  
> @@ -182,6 +185,26 @@ cmd_cp (char *src, char *dest)
>  }
>  
>  static void
> +cmd_cat (char *src)
> +{
> +  auto int cat_hook (grub_off_t ofs, char *buf, int len);
> +  int cat_hook (grub_off_t ofs, char *buf, int len)
> +  {
> +    (void) ofs;
> +
> +    if ((int) fwrite (buf, 1, len, stdout) != len)
> +      {
> +     grub_util_error (_("write error"));
> +     return 1;
> +      }
> +
> +    return 0;
> +  }
> +
> +  read_file (src, cat_hook);
> +}
> +
> +static void
>  cmd_cmp (char *src, char *dest)
>  {
>    FILE *ff;
> @@ -312,6 +335,9 @@ fstest (int n, char **args)
>      case CMD_CP:
>        cmd_cp (args[0], args[1]);
>        break;
> +    case CMD_CAT:
> +      cmd_cat (args[0]);
> +      break;
>      case CMD_CMP:
>        cmd_cmp (args[0], args[1]);
>        break;
> @@ -347,6 +373,7 @@ static struct argp_option options[] = {
>    {0,          0, 0      , OPTION_DOC, N_("Commands:"), 1},
>    {N_("ls PATH"),  0, 0      , OPTION_DOC, N_("List files in PATH."), 1},
>    {N_("cp FILE LOCAL"),  0, 0, OPTION_DOC, N_("Copy FILE to local file 
> LOCAL."), 1},
> +  {N_("cat FILE"), 0, 0      , OPTION_DOC, N_("Copy FILE to standard 
> output."), 1},
>    {N_("cmp FILE LOCAL"), 0, 0, OPTION_DOC, N_("Compare FILE with local file 
> LOCAL."), 1},
>    {N_("hex FILE"), 0, 0      , OPTION_DOC, N_("Hex dump FILE."), 1},
>    {N_("crc FILE"), 0, 0     , OPTION_DOC, N_("Get crc32 checksum of FILE."), 
> 1},
> @@ -459,6 +486,11 @@ argp_parser (int key, char *arg, struct
>         cmd = CMD_CP;
>            nparm = 2;
>       }
> +      else if (!grub_strcmp (arg, "cat"))
> +     {
> +       cmd = CMD_CAT;
> +       nparm = 1;
> +     }
>        else if (!grub_strcmp (arg, "cmp"))
>       {
>         cmd = CMD_CMP;
>
>   


-- 
Regards
Vladimir 'φ-coder/phcoder' Serbinenko

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