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
From ed8f419524fc81a378280579ec3c23af527d4772 Mon Sep 17 00:00:00 2001
From: Christian Franke
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
---
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, );
+ 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
