linux-kernel - Re: [PATCH v2] NFSD: Avoid clashing function prototypes

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Date:   Fri, 2 Dec 2022 21:41:33 +0000
From:   Chuck Lever III <chuck.lever@...cle.com>
To:     Kees Cook <keescook@...omium.org>
CC:     Jeff Layton <jlayton@...nel.org>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH v2] NFSD: Avoid clashing function prototypes



> On Dec 2, 2022, at 3:48 PM, Kees Cook <keescook@...omium.org> wrote:
> 
> When built with Control Flow Integrity, function prototypes between
> caller and function declaration must match. These mismatches are visible
> at compile time with the new -Wcast-function-type-strict in Clang[1].
> 
> There were 97 warnings produced by NFS. For example:
> 
> fs/nfsd/nfs4xdr.c:2228:17: warning: cast from '__be32 (*)(struct nfsd4_compoundargs *, struct nfsd4_access *)' (aka 'unsigned int (*)(struct nfsd4_compoundargs *, struct nfsd4_access *)') to 'nfsd4_dec' (aka 'unsigned int (*)(struct nfsd4_compoundargs *, void *)') converts to incompatible function type [-Wcast-function-type-strict]
>        [OP_ACCESS]             = (nfsd4_dec)nfsd4_decode_access,
>                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> The enc/dec callbacks were defined as passing "void *" as the second
> argument, but were being implicitly cast to a new type. Replace the
> argument with union nfsd4_op_u, and perform explicit member selection
> in the function body. There are no resulting binary differences.
> 
> Changes were made mechanically using the following Coccinelle script,
> with minor by-hand fixes for members that didn't already match their
> existing argument name:
> 
> @find@
> identifier func;
> type T, opsT;
> identifier ops, N;
> @@
> 
> opsT ops[] = {
> 	[N] = (T) func,
> };
> 
> @already_void@
> identifier find.func;
> identifier name;
> @@
> 
> func(...,
> -void
> +union nfsd4_op_u
> *name)
> {
> 	...
> }
> 
> @proto depends on !already_void@
> identifier find.func;
> type T;
> identifier name;
> position p;
> @@
> 
> func@p(...,
> 	T name
> ) {
> 	...
>   }
> 
> @script:python get_member@
> type_name << proto.T;
> member;
> @@
> 
> coccinelle.member = cocci.make_ident(type_name.split("_", 1)[1].split(' ',1)[0])
> 
> @convert@
> identifier find.func;
> type proto.T;
> identifier proto.name;
> position proto.p;
> identifier get_member.member;
> @@
> 
> func@p(...,
> -	T name
> +	union nfsd4_op_u *u
> ) {
> +	T name = &u->member;
> 	...
>   }
> 
> @cast@
> identifier find.func;
> type T, opsT;
> identifier ops, N;
> @@
> 
> opsT ops[] = {
> 	[N] =
> -	(T)
> 	func,
> };
> 
> Cc: Chuck Lever <chuck.lever@...cle.com>
> Cc: Jeff Layton <jlayton@...nel.org>
> Cc: Gustavo A. R. Silva <gustavoars@...nel.org>
> Cc: linux-nfs@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> v1: https://lore.kernel.org/lkml/20221007235406.2951724-1-keescook@chromium.org/
> v2: switch to using "union nfsd4_op_u" (chuck)
> ---
> fs/nfsd/nfs4xdr.c | 632 +++++++++++++++++++++++++++-------------------
> 1 file changed, 377 insertions(+), 255 deletions(-)

Lightly tested here and applied for v6.2. Many thanks!



--
Chuck Lever