Re: [PATCH 1/3] namei: add LOOKUP_DFD_ROOT to use dfd as root
From: Andrey Vagin <hidden>
Date: 2016-07-05 21:36:56
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On Fri, Jul 1, 2016 at 5:55 PM, Omar Sandoval [off-list ref] wrote:
On Tue, Jun 28, 2016 at 10:38:28AM -0700, Andrey Vagin wrote:quoted
The problem is that a pathname can contain absolute symlinks and now they are resolved relative to the current root. If we want to open a file in another mount namespaces and we have a file descriptor to its root directory, we probably want to resolve pathname in the target mount namespace. For this we add this new flag. If LOOKUP_DFD_ROOT is set, path_init() initializes nd->root and nd->path to the same value. Signed-off-by: Andrey Vagin <redacted>Hi, Andrey, Seems like a useful feature. Make sure to cc linux-api@vger.kernel.org for new userspace interfaces. One comment on the implementation below.
Hi Omar, Thank you for the patch. I have sent a new version with your changes. If there will not be other comments in a few days, I will resent a whole seires with linux-api@vger.kernel.org in CC. Thanks, Andrew
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--- fs/namei.c | 12 +++++++++++- include/linux/namei.h | 2 ++ 2 files changed, 13 insertions(+), 1 deletion(-)diff --git a/fs/namei.c b/fs/namei.c index 70580ab..5f08b69 100644 --- a/fs/namei.c +++ b/fs/namei.c@@ -2148,7 +2148,7 @@ static const char *path_init(struct nameidata *nd, unsigned flags) nd->path.dentry = NULL; nd->m_seq = read_seqbegin(&mount_lock); - if (*s == '/') { + if (*s == '/' && !(flags & LOOKUP_DFD_ROOT)) { if (flags & LOOKUP_RCU) rcu_read_lock(); set_root(nd);@@ -2174,6 +2174,11 @@ static const char *path_init(struct nameidata *nd, unsigned flags) get_fs_pwd(current->fs, &nd->path); nd->inode = nd->path.dentry->d_inode; } + if (flags & LOOKUP_DFD_ROOT) { + nd->root = nd->path; + if (!(flags & LOOKUP_RCU)) + path_get(&nd->root);You're not initializing nd->root_seq here. That means that if we end up going through unlazy_walk(), we're going to call legitimize_path() (and thus read_seqcount_retry()) with stack garbage, get a spurious ECHILD, and do an unnecessary restart of the path lookup instead of dropping into ref-walk mode.quoted
+ } return s; } else { /* Caller must check execute permissions on the starting path component */@@ -2202,6 +2207,11 @@ static const char *path_init(struct nameidata *nd, unsigned flags) nd->inode = nd->path.dentry->d_inode; } fdput(f); + if (flags & LOOKUP_DFD_ROOT) { + nd->root = nd->path; + if (!(flags & LOOKUP_RCU)) + path_get(&nd->root); + }Same here. The following should do the trick:diff --git a/fs/namei.c b/fs/namei.c index 9958b605e822..101d1fb8d3cb 100644 --- a/fs/namei.c +++ b/fs/namei.c@@ -2176,7 +2176,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags) } if (flags & LOOKUP_DFD_ROOT) { nd->root = nd->path; - if (!(flags & LOOKUP_RCU)) + if (flags & LOOKUP_RCU) + nd->root_seq = nd->seq; + else path_get(&nd->root); } return s;@@ -2209,7 +2211,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags) fdput(f); if (flags & LOOKUP_DFD_ROOT) { nd->root = nd->path; - if (!(flags & LOOKUP_RCU)) + if (flags & LOOKUP_RCU) + nd->root_seq = nd->seq; + else path_get(&nd->root); } return s; --Omar _______________________________________________ Containers mailing list Containers@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/containers