CODEWITHSUNDEEP
linuxlinux-kernelkernelcompatibility
user3857404
user3857404

Reputation: 113

64-bit kernel module crashing when called by 32-bit client

I've written a 64-bit kernel module that works fine for 64-bit apps, but crashes for 32-bit apps. I need to figure out how how to make the module detect if a client is a 32-bit app, and, if so manage the conversion between the 32-bit struct and 64-bit struct in my module so the system calls work.

When the module is used by a 32-bit app, the recvmsg() function crashes if the msg has cmsghdrs, because the cmsghdr struct has different size in a 32-bit app.

My module doesn't provide a 32-bit compatibility layer explicitly, even though the kernel is configured to support 32-bit applications in compatibility mode.

The module implements a new protocol, defines struct proto_ops, but I don't see compat_recv in the proto ops.

The module acts as a proxy between the client and another application (64-bit), wherein it fetches responses to client requests from the backend using netlink messages as opposed to making __sys_socket*() calls.

Upvotes: 1

Views: 188

Answers (1)

nowaqq
nowaqq

Reputation: 271

compat_recv is not needed in proto_ops. 32 bit userspace call to recvmsg in 64 bit kernel should go through compat_sys_recvmsg, which set MSG_CMSG_COMPAT flag. That flag makes ___sys_recvmsg() do necessary conversions before doing callback to your proto_ops and before returning to userspace.

References (in kernel tree):

  • Compatibility System Calls (Generic) chapter of: /Documentation/adding-syscalls.txt
  • /net/compat.c
  • /net/socket.c

Regards, Mateusz.

Upvotes: 3

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