Re: How to open a file by it's full path in UEFI

[Laszlo Ersek <lersek@redhat.com>]

On 09/27/16 12:46, GN Keshava wrote:
> Hi Laszlo,
> 
> Thank you for the answer. It was helpful.
> 
> Considering option #1, can you give some more details (a small example
> or any reference link would be helpful), how I can use Shell APIs in my
> C file (which will compile to my .efi app)?

Hmmm, I don't have hands-on experience with this, but you might want to
try the ShellOpenFileByName() function, from
"ShellPkg/Include/Library/ShellLib.h".

You can find examples for UEFI applications that use the Shell library with:

  git grep -w ShellLib -- '*inf'

or just grep the tree for ShellOpenFileByName().

See also "AppPkg/ReadMe.txt".

> Considering option #2, How I can find device path programatically from
> my C file?

The EFI_SHELL_PROTOCOL.GetDevicePathFromMap() member function seems
relevant -- it is specified in the UEFI Shell spec --, but I would
definitely try ShellOpenFileByName() first.

Thanks
Laszlo

> Thanks again for the help. :)
> With regards,
> Keshava
> 
> On Tue, 27 Sep 2016 at 15:19 Laszlo Ersek <lersek@redhat.com
> <mailto:lersek@redhat.com>> wrote:
> 
>     On 09/27/16 11:25, GN Keshava wrote:
>     > Hi Laszlo,
>     >
>     > Thanks for the reply. I meant I have complete file path. I believe the
>     > "device path" is different. Is it possible to obtain DevicePath
>     using my
>     > full file path?
> 
>     The pathname you seem to have (as "complete") is specific to a given
>     simple FS, so system-wide it cannot be considered complete (there can be
>     multiple filesystems).
> 
>     In your original email I missed that you started with "FS1:". Andrew's
>     answer covers that case.
> 
>     In summary, you can do three things:
>     - have a pathname that starts with FSx: (which is a shell-specific
>     mapping), and use Andrew's recommendation,
>     - have a complete UEFI device path, and then use what I recommended,
>     - have no information for selecting the filesystem (from the many
>     possible), and use your current iteration.
> 
>     Options #1 and #2 actually correspond to each other, considering
>     "expressive power" / information content (as long as you are in the
>     shell); please see the MAP shell command.
> 
>     Thanks
>     Laszlo
> 
>     > On Tue, 27 Sep 2016 at 14:46 Laszlo Ersek <lersek@redhat.com
>     <mailto:lersek@redhat.com>
>     > <mailto:lersek@redhat.com <mailto:lersek@redhat.com>>> wrote:
>     >
>     >     On 09/27/16 11:03, GN Keshava wrote:
>     >     > Hi all,
>     >     >
>     >     >
>     >     > I'm trying to open a file from my UEFI application. The path of
>     >     file is
>     >     >
>     >     > fs1:/myfolder/myfile.txt
>     >     >
>     >     > The code :
>     >     >
>     >     > efiStatus = bs->LocateHandleBuffer(ByProtocol,
>     >     >                                    &sfspGuid,
>     >     >                                    NULL,
>     >     >                                    &handleCount,
>     >     >                                    &handles);
>     >     >
>     >     > for (index = 0; index < (int)handleCount; ++ index)
>     >     > {
>     >     >     EFI_SIMPLE_FILE_SYSTEM_PROTOCOL* fs = NULL;
>     >     >
>     >     >     efiStatus = bs->HandleProtocol(
>     >     >         handles[index],
>     >     >         &sfspGuid,
>     >     >         (void**)&fs);
>     >     >
>     >     >     EFI_FILE_PROTOCOL* root = NULL;
>     >     >     ...
>     >     >     efiStatus = fs->OpenVolume(fs, &root);
>     >     >
>     >     >     EFI_FILE_PROTOCOL* token = NULL;
>     >     >
>     >     >     efiStatus = root->Open(
>     >     >         root,
>     >     >         &token,
>     >     >         L"myfolder\\myfile.txt",
>     >     >         EFI_FILE_MODE_READ,
>     >     >         EFI_FILE_READ_ONLY | EFI_FILE_HIDDEN | EFI_FILE_SYSTEM);
>     >     > }
>     >     >
>     >     > But using this method, I can only go through all the file system
>     >     handles
>     >     > and open each volume and try opening my file.
>     >     >
>     >     > But I want to give full path to my file and open it in it's
>     volume.
>     >     >
>     >     > How can I acheive this?
>     >     > Thanks.
>     >
>     >     If you have a complete device path, you can use
>     gBS->LocateDevicePath()
>     >     with gEfiSimpleFileSystemProtocolGuid, to locate the handle
>     with the
>     >     most specific device path (--> the longest device path prefix)
>     with the
>     >     simple FS protocol installed on it. Then you can check if the
>     remaining
>     >     device path (returned by the service) consist of nothing but
>     one File
>     >     Path Media Device Path node. If so, you can open the simple FS
>     protocol
>     >     on the handle found, then use that to open the file by pathname.
>     >
>     >     Laszlo
>     >
>