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Message-ID: <20240730-unwahr-tannenwald-ee6157a063a4@brauner>
Date: Tue, 30 Jul 2024 10:59:04 +0200
From: Christian Brauner <brauner@...nel.org>
To: Song Liu <songliubraving@...a.com>
Cc: Song Liu <song@...nel.org>, bpf <bpf@...r.kernel.org>,
Linux-Fsdevel <linux-fsdevel@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
Kernel Team <kernel-team@...a.com>, "andrii@...nel.org" <andrii@...nel.org>,
"eddyz87@...il.com" <eddyz87@...il.com>, "ast@...nel.org" <ast@...nel.org>,
"daniel@...earbox.net" <daniel@...earbox.net>, "martin.lau@...ux.dev" <martin.lau@...ux.dev>,
"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>, "jack@...e.cz" <jack@...e.cz>,
"kpsingh@...nel.org" <kpsingh@...nel.org>, "mattbobrowski@...gle.com" <mattbobrowski@...gle.com>
Subject: Re: [PATCH bpf-next 2/2] selftests/bpf: Add tests for
bpf_get_dentry_xattr
On Tue, Jul 30, 2024 at 05:58:31AM GMT, Song Liu wrote:
> Hi Christian,
>
> Thanks a lot for your detailed explanation! We will revisit the design
> based on these comments and suggestions.
>
> One more question about a potential new kfunc bpf_get_inode_xattr():
> Should it take dentry as input? IOW, should it look like:
>
> __bpf_kfunc int bpf_get_inode_xattr(struct dentry *dentry, const char *name__str,
> struct bpf_dynptr *value_p)
> {
> struct bpf_dynptr_kern *value_ptr = (struct bpf_dynptr_kern *)value_p;
> u32 value_len;
> void *value;
> int ret;
>
> if (strncmp(name__str, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN))
> return -EPERM;
>
> value_len = __bpf_dynptr_size(value_ptr);
> value = __bpf_dynptr_data_rw(value_ptr, value_len);
> if (!value)
> return -EINVAL;
>
> ret = inode_permission(&nop_mnt_idmap, inode, MAY_READ);
> if (ret)
> return ret;
> return __vfs_getxattr(dentry, inode, name__str, value, value_len);
> }
>
>
> I am asking because many security_inode_* hooks actually taking dentry as
> argument. So it makes sense to use dentry for kfuncs. Maybe we should
Some filesystems (i) require access to the @dentry in their xattr
handlers (e.g. 9p) and (ii) ->get() and ->set() xattr handlers can be
called when @inode hasn't been attached to @dentry yet.
So if you allowed to call bpf_get_*_xattr() from
security_d_instantiate() to somehow retrieve xattrs from there, then you
need to pass @dentry and @inode separately and you cannot use
@dentry->d_inode because it would still be NULL.
However, I doubt you'd call bpf_get_*_xattr() from
security_d_instantiate() so imo just pass the dentry and add a check
like:
struct inode *inode = d_inode(dentry);
if (WARN_ON(!inode))
return -EINVAL;
in there.
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