Re: [PATCH] idmapped-mounts: probe O_TMPFILE support with the caller's mapped id

friendly ping...

在 2026/7/31 17:02, Zizhi Wo 写道:
> From: Zizhi Wo <[email protected]>
> 
> Kernel commit 539dce114465 ("fs: refuse O_TMPFILE creation with an
> unmapped fsuid or fsgid") made vfs_tmpfile() return -EOVERFLOW when the
> caller's fsuid/fsgid has no mapping in the filesystem's user namespace
> after applying the mount's idmapping. This is a no-op on non-idmapped
> mounts and only takes effect on an idmapped mount that does not map the
> caller.
> 
> The idmapped-mount testcases behind generic/696 and generic/697 build an
> idmapping (e.g. "0 10000 10000") that covers host uid 10000-19999 but
> not uid 0, then call openat_tmpfile_supported() from the parent process
> that is still real root (fsuid 0). The probe now gets -EOVERFLOW, falls
> into the log_errno() branch, and prints
> 
>    utils.c: 928: openat_tmpfile_supported - Value too large for defined \
> data type - failure: create
> 
> to stderr. Both testcases expect "Silence is golden", so this spurious
> output fails them, even though the kernel behavior is correct and the
> testcases' own assertions all pass.
> 
> Move each openat_tmpfile_supported() call into the child, after
> switch_ids()/switch_userns(), so it probes with the mapped identity the
> tmpfile section uses. Silencing EOVERFLOW in openat_tmpfile_supported()
> would make it indistinguishable whether the idmapping genuinely has no
> mapping for any caller, or the probe just ran in the unmapped parent before
> the switch.
> 
> Signed-off-by: Zizhi Wo <[email protected]>
> ---
>   src/vfs/idmapped-mounts.c | 32 ++++++++++++++++++++------------
>   1 file changed, 20 insertions(+), 12 deletions(-)
> 
> diff --git a/src/vfs/idmapped-mounts.c b/src/vfs/idmapped-mounts.c
> index ed9992f9..bfbe9042 100644
> --- a/src/vfs/idmapped-mounts.c
> +++ b/src/vfs/idmapped-mounts.c
> @@ -3838,8 +3838,6 @@ int tcore_setgid_create_idmapped(const struct vfstest_info *info)
>   		goto out;
>   	}
>   
> -	supported = openat_tmpfile_supported(open_tree_fd);
> -
>   	pid = fork();
>   	if (pid < 0) {
>   		log_stderr("failure: fork");
> @@ -3849,6 +3847,8 @@ int tcore_setgid_create_idmapped(const struct vfstest_info *info)
>   		if (!switch_ids(10000, 11000))
>   			die("failure: switch fsids");
>   
> +		supported = openat_tmpfile_supported(open_tree_fd);
> +
>   		/* create regular file via open() */
>   		file1_fd = openat(open_tree_fd, FILE1, O_CREAT | O_EXCL | O_CLOEXEC, S_IXGRP | S_ISGID);
>   		if (file1_fd < 0)
> @@ -4014,8 +4014,6 @@ int tcore_setgid_create_idmapped_in_userns(const struct vfstest_info *info)
>   		goto out;
>   	}
>   
> -	supported = openat_tmpfile_supported(open_tree_fd);
> -
>   	pid = fork();
>   	if (pid < 0) {
>   		log_stderr("failure: fork");
> @@ -4025,6 +4023,8 @@ int tcore_setgid_create_idmapped_in_userns(const struct vfstest_info *info)
>   		if (!switch_userns(attr.userns_fd, 0, 0, false))
>   			die("failure: switch_userns");
>   
> +		supported = openat_tmpfile_supported(open_tree_fd);
> +
>   		/* create regular file via open() */
>   		file1_fd = openat(open_tree_fd, FILE1, O_CREAT | O_EXCL | O_CLOEXEC, S_IXGRP | S_ISGID);
>   		if (file1_fd < 0)
> @@ -4133,6 +4133,8 @@ int tcore_setgid_create_idmapped_in_userns(const struct vfstest_info *info)
>   		if (!switch_userns(attr.userns_fd, 0, 0, false))
>   			die("failure: switch_userns");
>   
> +		supported = openat_tmpfile_supported(open_tree_fd);
> +
>   		if (!caps_down_fsetid())
>   			die("failure: caps_down_fsetid");
>   
> @@ -4257,6 +4259,8 @@ int tcore_setgid_create_idmapped_in_userns(const struct vfstest_info *info)
>   		if (!switch_userns(attr.userns_fd, 0, 1000, false))
>   			die("failure: switch_userns");
>   
> +		supported = openat_tmpfile_supported(open_tree_fd);
> +
>   		if (!caps_down_fsetid())
>   			die("failure: caps_down_fsetid");
>   
> @@ -7733,8 +7737,6 @@ static int setgid_create_umask_idmapped(const struct vfstest_info *info)
>   		goto out;
>   	}
>   
> -	supported = openat_tmpfile_supported(open_tree_fd);
> -
>   	pid = fork();
>   	if (pid < 0) {
>   		log_stderr("failure: fork");
> @@ -7752,6 +7754,8 @@ static int setgid_create_umask_idmapped(const struct vfstest_info *info)
>   		if (!switch_ids(10000, 11000))
>   			die("failure: switch fsids");
>   
> +		supported = openat_tmpfile_supported(open_tree_fd);
> +
>   		/* create regular file via open() */
>   		file1_fd = openat(open_tree_fd, FILE1, O_CREAT | O_EXCL | O_CLOEXEC, S_IXGRP | S_ISGID);
>   		if (file1_fd < 0)
> @@ -7947,8 +7951,6 @@ static int setgid_create_umask_idmapped_in_userns(const struct vfstest_info *inf
>   		goto out;
>   	}
>   
> -	supported = openat_tmpfile_supported(open_tree_fd);
> -
>   	/*
>   	 * Below we verify that setgid inheritance for a newly created file or
>   	 * directory works correctly. As part of this we need to verify that
> @@ -7984,6 +7986,8 @@ static int setgid_create_umask_idmapped_in_userns(const struct vfstest_info *inf
>   		if (!switch_userns(attr.userns_fd, 0, 0, false))
>   			die("failure: switch_userns");
>   
> +		supported = openat_tmpfile_supported(open_tree_fd);
> +
>   		if (!caps_down_fsetid())
>   			die("failure: caps_down_fsetid");
>   
> @@ -8181,8 +8185,6 @@ static int setgid_create_acl_idmapped(const struct vfstest_info *info)
>   		goto out;
>   	}
>   
> -	supported = openat_tmpfile_supported(open_tree_fd);
> -
>   	pid = fork();
>   	if (pid < 0) {
>   		log_stderr("failure: fork");
> @@ -8207,6 +8209,8 @@ static int setgid_create_acl_idmapped(const struct vfstest_info *info)
>   		if (!switch_ids(10000, 11000))
>   			die("failure: switch fsids");
>   
> +		supported = openat_tmpfile_supported(open_tree_fd);
> +
>   		/* create regular file via open() */
>   		file1_fd = openat(open_tree_fd, FILE1, O_CREAT | O_EXCL | O_CLOEXEC, S_IXGRP | S_ISGID);
>   		if (file1_fd < 0)
> @@ -8342,6 +8346,8 @@ static int setgid_create_acl_idmapped(const struct vfstest_info *info)
>   		if (!switch_ids(10000, 11000))
>   			die("failure: switch fsids");
>   
> +		supported = openat_tmpfile_supported(open_tree_fd);
> +
>   		/* create regular file via open() */
>   		file1_fd = openat(open_tree_fd, FILE1, O_CREAT | O_EXCL | O_CLOEXEC, S_IXGRP | S_ISGID);
>   		if (file1_fd < 0)
> @@ -8536,8 +8542,6 @@ static int setgid_create_acl_idmapped_in_userns(const struct vfstest_info *info)
>   		goto out;
>   	}
>   
> -	supported = openat_tmpfile_supported(open_tree_fd);
> -
>   	/*
>   	 * Below we verify that setgid inheritance for a newly created file or
>   	 * directory works correctly. As part of this we need to verify that
> @@ -8580,6 +8584,8 @@ static int setgid_create_acl_idmapped_in_userns(const struct vfstest_info *info)
>   		if (!switch_userns(attr.userns_fd, 0, 0, false))
>   			die("failure: switch_userns");
>   
> +		supported = openat_tmpfile_supported(open_tree_fd);
> +
>   		if (!caps_down_fsetid())
>   			die("failure: caps_down_fsetid");
>   
> @@ -8723,6 +8729,8 @@ static int setgid_create_acl_idmapped_in_userns(const struct vfstest_info *info)
>   		if (!switch_userns(attr.userns_fd, 0, 0, false))
>   			die("failure: switch_userns");
>   
> +		supported = openat_tmpfile_supported(open_tree_fd);
> +
>   		if (!caps_down_fsetid())
>   			die("failure: caps_down_fsetid");
>