[PATCH] Cygwin: Fix access to block devices below /proc/sys.

[Christian Franke]

There a very few but occasionally interesting use cases for read access 
to block devices below /proc/sys:

- Read raw images behind drive letters which are not linked to regular 
/dev/sdXN partitions. For example read decrypted images of VeraCrypt 
partitions or container files:
/proc/sys/DosDevices/X: -> /proc/sys/Device/VeraCryptVolumeX

- Read raw images of Volume Shadow Copies:
/proc/sys/Device/HarddiskVolumeShadowCopy*

Copying such an image actually works with 'dd', but 'ddrescue' reports a 
non seekable device. This is because fhandler_virtual::lseek() is used. 
It calls fhandler_procsys::fill_filebuf() which does not make any sense 
in this context. This lseek() always fails - without setting errno, BTW.

The attached experimental patch does not fix the lseek() (sorry), but 
handles such block devices with fhandler_dev_floppy instead. Tested with 
above use cases.

I'm not sure whether this could break access to other /proc/sys block 
devices. This would happen if fh->exists() returns virt_blk for devices 
which do not support IOCTL_DISK_GET_DRIVE_GEOMETRY* or 
IOCTL_DISK_GET_PARTITION_INFO*.

Regards,
Christian

-------------- next part --------------
From ed8f419524fc81a378280579ec3c23af527d4772 Mon Sep 17 00:00:00 2001
From: Christian Franke <franke@computer.org>
Date: Sat, 28 Nov 2020 22:09:23 +0100
Subject: [PATCH] Cygwin: Fix access to block devices below /proc/sys.

Use fhandler_dev_floppy instead of fhandler_procsys for such devices.
The read()/write() functions from fhandler_procsys do not ensure
sector aligned transfers and lseek() fails always.

Signed-off-by: Christian Franke <franke@computer.org>
---
 winsup/cygwin/path.cc | 29 +++++++++++++++++++----------
 1 file changed, 19 insertions(+), 10 deletions(-)

diff --git a/winsup/cygwin/path.cc b/winsup/cygwin/path.cc
index 4f5f03a76..7e6243d32 100644
--- a/winsup/cygwin/path.cc
+++ b/winsup/cygwin/path.cc
@@ -863,19 +863,28 @@ path_conv::check (const char *src, unsigned opt,
 			dev.parse (FH_FS);
 			goto is_fs_via_procsys;
 		      case virt_blk:
-			/* Block special device.  If the trailing slash has been
-			   requested, the target is the root directory of the
-			   filesystem on this block device.  So we convert this
-			   to a real file and attach the backslash. */
-			if (component == 0 && need_directory)
+			/* Block special device.  Convert to a /dev/sd* like
+			   block device unless the trailing slash has been
+			   requested.  In this case, the target is the root
+			   directory of the filesystem on this block device.
+			   So we convert this to a real file and attach the
+			   backslash. */
+			if (component == 0)
 			  {
-			    dev.parse (FH_FS);
-			    strcat (full_path, "\\");
-			    fileattr = FILE_ATTRIBUTE_DIRECTORY
-				       | FILE_ATTRIBUTE_DEVICE;
+			    fileattr = FILE_ATTRIBUTE_DEVICE;
+			    if (!need_directory)
+			      /* Use a /dev/sd* device number > /dev/sddx.
+				 FIXME: Define a new major DEV_ice number. */
+			      dev.parse (DEV_SD_HIGHPART_END, 9999);
+			    else
+			      {
+				dev.parse (FH_FS);
+				strcat (full_path, "\\");
+				fileattr |= FILE_ATTRIBUTE_DIRECTORY;
+			      }
 			    goto out;
 			  }
-			fallthrough;
+			break;
 		      case virt_chr:
 			if (component == 0)
 			  fileattr = FILE_ATTRIBUTE_DEVICE;
-- 
2.29.2