Thanks Valdis. Yes , custom kernel module has struct proto_ops pointing to the routines in the module. my kernel module routines do not cal any of linux provided socket* calls. it gets data from different app and provide that data back to app calling socket* APIs. i understand linux kernel calls 32 bit version of syscall handler from sys call table . https://kernel.org/doc/html/latest/process/adding-syscalls.html but how do i do this in custom kernel module. how to differentiate 32 bit and 64 app in my kernel module. do i need all stuff from compat.h and compat.c (if i use same files will that work) ? do i need to change some files in std kernel files too like syscall_32.tbl file , and recompile kernel ? It would be great if there is any example / any web links for this to understad Thanks On Wed, Jan 4, 2017 at 10:10 AM, <Valdis.Kletnieks@vt.edu> wrote:

On Wed, 04 Jan 2017 09:34:58 +0530, Pradeepa Kumar said:

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I have a custom kernel module providing new protocol and providing socket system calls.

In general, a loadable module can provide a new protocol, but can't add new syscalls. However, adding support for socket(), connect(), recvmsg() and so on for a new protocol *can* be done from a module, as long as the protocol provides a suitable struct proto_ops pointing to the routines in the module.

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i am seeing issues when 32 bit app runs (for exp recvmsg() call does not work if msg has cmsghdrs as struct size of cmsghdr is different in 32 bit and 64 bit).

That's a good reason to double-check your code to ensure that you don't make that same mistake.

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This is because my custom kernel module does not have 32 bit compatibility layer ( but linux kernel has this in compat.c etc).

Hold that thought.

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is it simple to add compatibility layer to my custom kernel module

Given your previous sentence, you should be able to figure that out.

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how do i do this (any links ?)

/* * linux/kernel/compat.c * * Kernel compatibililty routines for e.g. 32 bit syscall support * on 64 bit kernels. * * Copyright (C) 2002-2003 Stephen Rothwell, IBM Corporation

May already have the functions you need. If not, use the code as a guide to writing the stuff you're still lacking.

Note that in particular, recvmsg() combatability is already done for you via this chunk of code in net/compat.c:

COMPAT_SYSCALL_DEFINE3(recvmsg, int, fd, struct compat_msghdr __user *, msg, unsigned int, flags) { return __sys_recvmsg(fd, (struct user_msghdr __user *)msg, flags | MSG_CMSG_COMPAT); }

Go look and see what the MSG_CMSG_COMPAT flag does if you want the gory details.