Add server support for the Linux NFSv3 extended attribute side protocol (XATTR). Signed-off-by: James Morris <jmorris@xxxxxxxxx> --- fs/nfsd/Kconfig | 8 + fs/nfsd/Makefile | 1 + fs/nfsd/nfs3xattr.c | 354 ++++++++++++++++++++++++++++++++++++++++++++ fs/nfsd/nfsctl.c | 3 + fs/nfsd/nfssvc.c | 60 +++++++- fs/nfsd/vfs.c | 5 +- fs/nfsd/vfs.h | 13 ++ fs/nfsd/xdr3.h | 46 ++++++ include/linux/sunrpc/svc.h | 2 +- 9 files changed, 486 insertions(+), 6 deletions(-) create mode 100644 fs/nfsd/nfs3xattr.c diff --git a/fs/nfsd/Kconfig b/fs/nfsd/Kconfig index 503b9da..4252d16 100644 --- a/fs/nfsd/Kconfig +++ b/fs/nfsd/Kconfig @@ -64,6 +64,14 @@ config NFSD_V3_ACL If unsure, say N. +config NFSD_V3_XATTR + bool "NFS server support for the NFSv3 XATTR protocol extension (EXPERIMENTAL)" + depends on NFSD_V3 && EXPERIMENTAL + help + NFS server support for the NFSv3 XATTR protocol. + + If unsure, say N. + config NFSD_V4 bool "NFS server support for NFS version 4 (EXPERIMENTAL)" depends on NFSD && PROC_FS && EXPERIMENTAL diff --git a/fs/nfsd/Makefile b/fs/nfsd/Makefile index 9b118ee..e206b52 100644 --- a/fs/nfsd/Makefile +++ b/fs/nfsd/Makefile @@ -9,5 +9,6 @@ nfsd-y := nfssvc.o nfsctl.o nfsproc.o nfsfh.o vfs.o \ nfsd-$(CONFIG_NFSD_V2_ACL) += nfs2acl.o nfsd-$(CONFIG_NFSD_V3) += nfs3proc.o nfs3xdr.o nfsd-$(CONFIG_NFSD_V3_ACL) += nfs3acl.o +nfsd-$(CONFIG_NFSD_V3_XATTR) += nfs3xattr.o nfsd-$(CONFIG_NFSD_V4) += nfs4proc.o nfs4xdr.o nfs4state.o nfs4idmap.o \ nfs4acl.o nfs4callback.o nfs4recover.o diff --git a/fs/nfsd/nfs3xattr.c b/fs/nfsd/nfs3xattr.c new file mode 100644 index 0000000..b5a5faa --- /dev/null +++ b/fs/nfsd/nfs3xattr.c @@ -0,0 +1,354 @@ +/* + * Process version 3 NFSXATTR requests. + * + * Based on the NFSACL code by: + * Copyright (C) 2002-2003 Andreas Gruenbacher <agruen@xxxxxxx> + * + * Copyright (C) 2009 Red Hat, Inc., James Morris <jmorris@xxxxxxxxxx> + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License version 2, + * as published by the Free Software Foundation. + */ +#include <linux/sunrpc/svc.h> +#include <linux/nfs3.h> +#include <linux/xattr.h> +#include <linux/nfs_xattr.h> + +#include "nfsd.h" +#include "xdr3.h" +#include "vfs.h" +#include "cache.h" + +#define NFSDDBG_FACILITY NFSDDBG_PROC +#define RETURN_STATUS(st) { resp->status = (st); return (st); } + +/* NULL call */ +static __be32 nfsd3_proc_null(struct svc_rqst *rqstp, void *argp, void *resp) +{ + return nfs_ok; +} + +/* + * GETXATTR + * + * FIXME: + * - Implement shared xattr cache + * - Audit nfs error returns + */ +static __be32 nfsd3_proc_getxattr(struct svc_rqst * rqstp, + struct nfsd3_getxattrargs *argp, + struct nfsd3_getxattrres *resp) +{ + __be32 nfserr = nfserrno(-EINVAL); + svc_fh *fh; + void *value; + int ret; + char *name, *xattr_name = argp->xattr_name; + unsigned int size_max = argp->xattr_size_max; + unsigned int name_len = argp->xattr_name_len; + + dprintk("nfsd: GETXATTR(3) %s %.*s %u\n", SVCFH_fmt(&argp->fh), + name_len, xattr_name, size_max); + + if (name_len > XATTR_NAME_MAX) + RETURN_STATUS(nfserr); + + if (size_max > XATTR_SIZE_MAX) + RETURN_STATUS(nfserr); + + /* Probes must be handled by the client */ + if (size_max == 0) + RETURN_STATUS(nfserr); + + fh = fh_copy(&resp->fh, &argp->fh); + nfserr = fh_verify(rqstp, &resp->fh, 0, NFSD_MAY_READ); + if (nfserr) + RETURN_STATUS(nfserr); + + /* Convert xdr string to real string */ + name = kmalloc(name_len + 1, GFP_KERNEL); + if (name == NULL) + RETURN_STATUS(nfserrno(-ENOMEM)); + + ret = snprintf(name, name_len + 1, "%.*s", name_len, xattr_name); + if (ret > name_len) { + nfserr = nfserrno(-EINVAL); + goto cleanup; + } + + /* Only the user namespace is currently supported by the server */ + if (strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) { + nfserr = nfserrno(-EINVAL); + goto cleanup; + } + + ret = nfsd_getxattr(fh->fh_dentry, name, &value); + if (ret <= 0) { + if (ret == 0) + ret = -ENODATA; + nfserr = nfserrno(ret); + goto cleanup; + } + + nfserr = 0; + resp->xattr_val = value; + resp->xattr_val_len = ret; + +cleanup: + kfree(name); + RETURN_STATUS(nfserr); +} + +/* cribbed from decode pathname */ +static __be32 *decode_xattrname(__be32 *p, char **namp, unsigned int *lenp) +{ + char *name; + unsigned int i; + + p = xdr_decode_string_inplace(p, namp, lenp, XATTR_NAME_MAX); + if (p != NULL) + for (i = 0, name = *namp; i < *lenp; i++, name++) + if (*name == '\0') + return NULL; + return p; +} + +static int nfs3svc_decode_getxattrargs(struct svc_rqst *rqstp, __be32 *p, + struct nfsd3_getxattrargs *argp) +{ + if (!(p = nfs3svc_decode_fh(p, &argp->fh))) + return 0; + if (!(p = decode_xattrname(p, &argp->xattr_name, &argp->xattr_name_len))) + return 0; + argp->xattr_size_max = ntohl(*p++); + return xdr_argsize_check(rqstp, p); +} + +static int nfs3svc_encode_getxattrres(struct svc_rqst *rqstp, __be32 *p, + struct nfsd3_getxattrres *resp) +{ + p = nfs3svc_encode_post_op_attr(rqstp, p, &resp->fh); + if (resp->status == 0) + p = xdr_encode_array(p, resp->xattr_val, resp->xattr_val_len); + return xdr_ressize_check(rqstp, p); +} + +static int nfs3svc_release_getxattr(struct svc_rqst *rqstp, __be32 *p, + struct nfsd3_getxattrres *resp) +{ + fh_put(&resp->fh); + kfree(resp->xattr_val); + return 1; +} + +/* + * SETXATTR and RMXATTR + * + * RMXATTR is detected with zero buffer len and XATTR_REPLACE. + * + */ +static __be32 nfsd3_proc_setxattr(struct svc_rqst * rqstp, + struct nfsd3_setxattrargs *argp, + struct nfsd3_setxattrres *resp) +{ + __be32 nfserr = nfserrno(-EINVAL); + svc_fh *fh; + int ret; + char *name, *xattr_name = argp->xattr_name; + unsigned int name_len = argp->xattr_name_len; + unsigned int val_len = argp->xattr_val_len; + unsigned int flags = argp->xattr_flags; + + dprintk("nfsd: SETXATTR(3) %s %.*s %u %#x\n", SVCFH_fmt(&argp->fh), + name_len, xattr_name, val_len, flags); + + if (name_len > XATTR_NAME_MAX) + RETURN_STATUS(nfserr); + + if (val_len > XATTR_SIZE_MAX) + RETURN_STATUS(nfserr); + + if (flags & ~(XATTR_CREATE|XATTR_REPLACE)) + RETURN_STATUS(nfserr); + + fh = fh_copy(&resp->fh, &argp->fh); + nfserr = fh_verify(rqstp, &resp->fh, 0, NFSD_MAY_SATTR); + if (nfserr) + RETURN_STATUS(nfserr); + + /* Convert xdr string to real string */ + name = kmalloc(name_len + 1, GFP_KERNEL); + if (name == NULL) + RETURN_STATUS(nfserrno(-ENOMEM)); + + ret = snprintf(name, name_len + 1, "%.*s", name_len, xattr_name); + if (ret > name_len) { + nfserr = nfserrno(-EINVAL); + goto cleanup; + } + + /* Only the user namespace is currently supported by the server */ + if (strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) { + nfserr = nfserrno(-EINVAL); + goto cleanup; + } + + if (!val_len) { + if (flags & ~XATTR_REPLACE) { + nfserr = nfserrno(-EINVAL); + goto cleanup; + } + ret = vfs_removexattr(fh->fh_dentry, name); + } else + ret = vfs_setxattr(fh->fh_dentry, name, + argp->xattr_val, val_len, flags); + + nfserr = nfserrno(ret); + +cleanup: + kfree(name); + RETURN_STATUS(nfserr); +} + +static int nfs3svc_decode_setxattrargs(struct svc_rqst *rqstp, __be32 *p, + struct nfsd3_setxattrargs *argp) +{ + if (!(p = nfs3svc_decode_fh(p, &argp->fh))) + return 0; + if (!(p = decode_xattrname(p, &argp->xattr_name, &argp->xattr_name_len))) + return 0; + if (!(p = xdr_decode_string_inplace(p, &argp->xattr_val, + &argp->xattr_val_len, XATTR_SIZE_MAX))) + return 0; + argp->xattr_flags = ntohl(*p++); + return xdr_argsize_check(rqstp, p); +} + +static int nfs3svc_encode_setxattrres(struct svc_rqst *rqstp, __be32 *p, + struct nfsd3_setxattrres *resp) +{ + p = nfs3svc_encode_post_op_attr(rqstp, p, &resp->fh); + return xdr_ressize_check(rqstp, p); +} + +static int nfs3svc_release_setxattr(struct svc_rqst *rqstp, __be32 *p, + struct nfsd3_setxattrres *resp) +{ + fh_put(&resp->fh); + return 1; +} + +/* + * LISTXATTR + * + * TODO: namespace filtering? + */ +static __be32 nfsd3_proc_listxattr(struct svc_rqst * rqstp, + struct nfsd3_listxattrargs *argp, + struct nfsd3_listxattrres *resp) +{ + __be32 nfserr = nfserrno(-EINVAL); + svc_fh *fh; + char *list; + int ret; + unsigned int list_max = argp->xattr_list_max; + + dprintk("nfsd: LISTXATTR(3) %s %u\n", SVCFH_fmt(&argp->fh), list_max); + + if (list_max > XATTR_LIST_MAX) + RETURN_STATUS(nfserr); + + /* Probes must be handled by the client */ + if (list_max == 0) + RETURN_STATUS(nfserr); + + fh = fh_copy(&resp->fh, &argp->fh); + nfserr = fh_verify(rqstp, &resp->fh, 0, NFSD_MAY_READ); + if (nfserr) + RETURN_STATUS(nfserr); + + list = kmalloc(list_max, GFP_ATOMIC); + if (list == NULL) + RETURN_STATUS(nfserrno(-ENOMEM)); + + ret = vfs_listxattr(fh->fh_dentry, list, list_max); + if (ret <= 0) { + if (ret == 0) + ret = -ENODATA; + RETURN_STATUS(nfserrno(ret)); + } + + nfserr = 0; + resp->xattr_list = list; + resp->xattr_list_len = ret; + + RETURN_STATUS(nfserr); +} + +static int nfs3svc_decode_listxattrargs(struct svc_rqst *rqstp, __be32 *p, + struct nfsd3_listxattrargs *argp) +{ + if (!(p = nfs3svc_decode_fh(p, &argp->fh))) + return 0; + argp->xattr_list_max = ntohl(*p++); + return xdr_argsize_check(rqstp, p); +} + +static int nfs3svc_encode_listxattrres(struct svc_rqst *rqstp, __be32 *p, + struct nfsd3_listxattrres *resp) +{ + p = nfs3svc_encode_post_op_attr(rqstp, p, &resp->fh); + if (resp->status == 0) + p = xdr_encode_array(p, resp->xattr_list, resp->xattr_list_len); + return xdr_ressize_check(rqstp, p); +} + +static int nfs3svc_release_listxattr(struct svc_rqst *rqstp, __be32 *p, + struct nfsd3_listxattrres *resp) +{ + fh_put(&resp->fh); + kfree(resp->xattr_list); + return 1; +} + +#define ST 1 /* status */ +#define AT 21 /* attributes */ +#define pAT (1+AT) /* post attributes - conditional */ + +#define nfs3svc_decode_voidargs NULL +#define nfs3svc_release_void NULL +#define nfsd3_voidres nfsd3_voidargs +struct nfsd3_voidargs { int dummy; }; + +#define PROC(name, argt, rest, relt, cache, respsize) \ + { (svc_procfunc) nfsd3_proc_##name, \ + (kxdrproc_t) nfs3svc_decode_##argt##args, \ + (kxdrproc_t) nfs3svc_encode_##rest##res, \ + (kxdrproc_t) nfs3svc_release_##relt, \ + sizeof(struct nfsd3_##argt##args), \ + sizeof(struct nfsd3_##rest##res), \ + 0, \ + cache, \ + respsize, \ + } + +#define G_RSZ (ST+pAT+1+(XATTR_SIZE_MAX>>2)) +#define S_RSZ (ST+pAT) +#define L_RSZ (ST+pAT+1+(XATTR_LIST_MAX>>2)) + +static struct svc_procedure nfsd_xattr_procedures3[] = { + PROC(null, void, void, void, RC_NOCACHE, ST), + PROC(getxattr, getxattr, getxattr, getxattr, RC_NOCACHE, G_RSZ), + PROC(setxattr, setxattr, setxattr, setxattr, RC_NOCACHE, S_RSZ), + PROC(listxattr, listxattr, listxattr, listxattr, RC_NOCACHE, L_RSZ), +}; + +struct svc_version nfsd_xattr_version3 = { + .vs_vers = 3, + .vs_nproc = 4, + .vs_proc = nfsd_xattr_procedures3, + .vs_dispatch = nfsd_dispatch, + .vs_xdrsize = NFS3_SVC_XDRSIZE, + .vs_hidden = 1, +}; diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c index 508941c..64945b5 100644 --- a/fs/nfsd/nfsctl.c +++ b/fs/nfsd/nfsctl.c @@ -1421,6 +1421,8 @@ static int create_proc_exports_entry(void) } #endif +extern void __init nfsd_prog_init(void); + static int __init init_nfsd(void) { int retval; @@ -1429,6 +1431,7 @@ static int __init init_nfsd(void) retval = nfs4_state_init(); /* nfs4 locking state */ if (retval) return retval; + nfsd_prog_init(); nfsd_stat_init(); /* Statistics */ retval = nfsd_reply_cache_init(); if (retval) diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c index 06b2a26..29096ae 100644 --- a/fs/nfsd/nfssvc.c +++ b/fs/nfsd/nfssvc.c @@ -15,6 +15,7 @@ #include <linux/sunrpc/svcsock.h> #include <linux/lockd/bind.h> #include <linux/nfsacl.h> +#include <linux/nfs_xattr.h> #include <linux/seq_file.h> #include "nfsd.h" #include "cache.h" @@ -87,6 +88,27 @@ static struct svc_stat nfsd_acl_svcstats = { }; #endif /* defined(CONFIG_NFSD_V2_ACL) || defined(CONFIG_NFSD_V3_ACL) */ +#ifdef CONFIG_NFSD_V3_XATTR +static struct svc_stat nfsd_xattr_svcstats; +static struct svc_version * nfsd_xattr_version[] = { + [3] = &nfsd_xattr_version3, +}; + +#define NFSD_XATTR_MINVERS 3 +#define NFSD_XATTR_NRVERS ARRAY_SIZE(nfsd_xattr_version) +static struct svc_version *nfsd_xattr_versions[NFSD_XATTR_NRVERS]; + +static struct svc_program nfsd_xattr_program = { + .pg_prog = NFS_XATTR_PROGRAM, + .pg_nvers = NFSD_XATTR_NRVERS, + .pg_vers = nfsd_xattr_versions, + .pg_name = "nfsxattr", + .pg_class = "nfsd", /* share nfsd auth */ + .pg_stats = &nfsd_xattr_svcstats, + .pg_authenticate = &svc_set_client, +}; +#endif /* CONFIG_NFSD_V3_XATTR */ + static struct svc_version * nfsd_version[] = { [2] = &nfsd_version2, #if defined(CONFIG_NFSD_V3) @@ -102,9 +124,6 @@ static struct svc_version * nfsd_version[] = { static struct svc_version *nfsd_versions[NFSD_NRVERS]; struct svc_program nfsd_program = { -#if defined(CONFIG_NFSD_V2_ACL) || defined(CONFIG_NFSD_V3_ACL) - .pg_next = &nfsd_acl_program, -#endif .pg_prog = NFS_PROGRAM, /* program number */ .pg_nvers = NFSD_NRVERS, /* nr of entries in nfsd_version */ .pg_vers = nfsd_versions, /* version table */ @@ -115,6 +134,28 @@ struct svc_program nfsd_program = { }; +static void __init nfsd_prog_add(struct svc_program *new) +{ + struct svc_program *p = &nfsd_program; + + while (p->pg_next) + p = p->pg_next; + + p->pg_next = new; +} + +/* Dynamically initialize list of service programs */ +void __init nfsd_prog_init(void) +{ +#if defined(CONFIG_NFSD_V2_ACL) || defined(CONFIG_NFSD_V3_ACL) + nfsd_prog_add(&nfsd_acl_program); +#endif + +#ifdef CONFIG_NFSD_V3_XATTR + nfsd_prog_add(&nfsd_xattr_program); +#endif +} + u32 nfsd_supported_minorversion; int nfsd_vers(int vers, enum vers_op change) @@ -128,6 +169,10 @@ int nfsd_vers(int vers, enum vers_op change) if (vers < NFSD_ACL_NRVERS) nfsd_acl_versions[vers] = nfsd_acl_version[vers]; #endif +#ifdef CONFIG_NFSD_V3_XATTR + if (vers < NFSD_XATTR_NRVERS) + nfsd_xattr_versions[vers] = nfsd_xattr_version[vers]; +#endif break; case NFSD_CLEAR: nfsd_versions[vers] = NULL; @@ -135,6 +180,10 @@ int nfsd_vers(int vers, enum vers_op change) if (vers < NFSD_ACL_NRVERS) nfsd_acl_versions[vers] = NULL; #endif +#ifdef CONFIG_NFSD_V3_XATTR + if (vers < NFSD_XATTR_NRVERS) + nfsd_xattr_versions[vers] = NULL; +#endif break; case NFSD_TEST: return nfsd_versions[vers] != NULL; @@ -213,6 +262,11 @@ void nfsd_reset_versions(void) nfsd_acl_program.pg_vers[i] = nfsd_acl_version[i]; #endif +#ifdef CONFIG_NFSD_V3_XATTR + for (i = NFSD_XATTR_MINVERS; i < NFSD_XATTR_NRVERS; i++) + nfsd_xattr_program.pg_vers[i] = + nfsd_xattr_version[i]; +#endif } } diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c index 3c11112..86309d2 100644 --- a/fs/nfsd/vfs.c +++ b/fs/nfsd/vfs.c @@ -454,8 +454,9 @@ out_nfserr: #if defined(CONFIG_NFSD_V2_ACL) || \ defined(CONFIG_NFSD_V3_ACL) || \ - defined(CONFIG_NFSD_V4) -static ssize_t nfsd_getxattr(struct dentry *dentry, char *key, void **buf) + defined(CONFIG_NFSD_V4) || \ + defined(CONFIG_NFSD_V3_XATTR) +ssize_t nfsd_getxattr(struct dentry *dentry, char *key, void **buf) { ssize_t buflen; ssize_t ret; diff --git a/fs/nfsd/vfs.h b/fs/nfsd/vfs.h index 217a62c..4a4d6ec 100644 --- a/fs/nfsd/vfs.h +++ b/fs/nfsd/vfs.h @@ -99,4 +99,17 @@ struct posix_acl *nfsd_get_posix_acl(struct svc_fh *, int); int nfsd_set_posix_acl(struct svc_fh *, int, struct posix_acl *); #endif +#if defined(CONFIG_NFSD_V2_ACL) || \ + defined(CONFIG_NFSD_V3_ACL) || \ + defined(CONFIG_NFSD_V4) || \ + defined(CONFIG_NFSD_V3_XATTR) +ssize_t nfsd_getxattr(struct dentry *dentry, char *key, void **buf); +#endif + +#ifdef CONFIG_NFSD_V3_XATTR +extern struct svc_version nfsd_xattr_version3; +#else +#define nfsd_xattr_version3 NULL +#endif + #endif /* LINUX_NFSD_VFS_H */ diff --git a/fs/nfsd/xdr3.h b/fs/nfsd/xdr3.h index 7df980e..e6ccc60 100644 --- a/fs/nfsd/xdr3.h +++ b/fs/nfsd/xdr3.h @@ -119,6 +119,27 @@ struct nfsd3_setaclargs { struct posix_acl *acl_default; }; +struct nfsd3_getxattrargs { + struct svc_fh fh; + char * xattr_name; + unsigned int xattr_name_len; + unsigned int xattr_size_max; +}; + +struct nfsd3_setxattrargs { + struct svc_fh fh; + unsigned int xattr_flags; + char * xattr_name; + unsigned int xattr_name_len; + char * xattr_val; + int xattr_val_len; +}; + +struct nfsd3_listxattrargs { + struct svc_fh fh; + unsigned int xattr_list_max; +}; + struct nfsd3_attrstat { __be32 status; struct svc_fh fh; @@ -227,6 +248,25 @@ struct nfsd3_getaclres { struct posix_acl *acl_default; }; +struct nfsd3_getxattrres { + __be32 status; + struct svc_fh fh; + char * xattr_val; + unsigned int xattr_val_len; +}; + +struct nfsd3_setxattrres { + __be32 status; + struct svc_fh fh; +}; + +struct nfsd3_listxattrres { + __be32 status; + struct svc_fh fh; + char * xattr_list; + unsigned int xattr_list_len; +}; + /* dummy type for release */ struct nfsd3_fhandle_pair { __u32 dummy; @@ -247,6 +287,9 @@ union nfsd3_xdrstore { struct nfsd3_linkargs linkargs; struct nfsd3_symlinkargs symlinkargs; struct nfsd3_readdirargs readdirargs; + struct nfsd3_getxattrargs getxattrargs; + struct nfsd3_setxattrargs setxattrargs; + struct nfsd3_listxattrargs listxattrargs; struct nfsd3_diropres diropres; struct nfsd3_accessres accessres; struct nfsd3_readlinkres readlinkres; @@ -260,6 +303,9 @@ union nfsd3_xdrstore { struct nfsd3_pathconfres pathconfres; struct nfsd3_commitres commitres; struct nfsd3_getaclres getaclres; + struct nfsd3_getxattrres getxattrres; + struct nfsd3_setxattrres setxattrres; + struct nfsd3_listxattrres listxattrres; }; #define NFS3_SVC_XDRSIZE sizeof(union nfsd3_xdrstore) diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h index 5a3085b..8bde5c1 100644 --- a/include/linux/sunrpc/svc.h +++ b/include/linux/sunrpc/svc.h @@ -371,7 +371,7 @@ struct svc_version { u32 vs_xdrsize; /* xdrsize needed for this version */ unsigned int vs_hidden : 1; /* Don't register with portmapper. - * Only used for nfsacl so far. */ + * Used for nfsacl and nfsxattr. */ /* Override dispatch function (e.g. when caching replies). * A return value of 0 means drop the request. -- 1.7.0.1 -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html