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    Subjectexec: Move unshare_files and guarantee files_struct.count is correct


    A while ago it was reported that posix file locking goes wrong when a
    multi-threaded process calls exec. I looked into the history and this
    is definitely a regression, that should be fixed if we can.

    This set of changes cleanups of the code in exec so hopefully this code
    will not regress again. Then it adds helpers and fixes the users of
    files_struct so the reference count is only incremented if COPY_FILES is
    passed to clone. Then it removes helpers (get_files_struct,
    __install_fd, __alloc_fd, __close_fd) that are no longer needed and
    if used would encourage code that increments the count of files_struct
    somewhere besides in clone when COPY_FILES is passed.

    In addition to fixing the bug in exec and simplifing the code this set
    of changes by virtue of getting files_struct.count correct it optimizes
    fdget. With proc and other places not temporarily increasing the count
    on files_struct __fget_light should succeed more often in being able to
    return a struct file without touching it's reference count.

    Fixing the count in files_struct was suggested by Oleg[1].

    For those that are interested in the history of this issue I have
    included as much of it as I could find in the first change.

    fs/coredump.c | 5 +--
    fs/exec.c | 26 ++++++-----
    fs/file.c | 110 +++++++++++++++++++++--------------------------
    fs/open.c | 2 +-
    fs/proc/fd.c | 47 +++++++-------------
    include/linux/fdtable.h | 13 ++----
    include/linux/syscalls.h | 6 +--
    kernel/bpf/syscall.c | 20 ++-------
    kernel/bpf/task_iter.c | 43 +++++-------------
    kernel/fork.c | 12 +++---
    kernel/kcmp.c | 20 ++-------
    11 files changed, 109 insertions(+), 195 deletions(-)

    Eric W. Biederman (17):
    exec: Move unshare_files to fix posix file locking during exec
    exec: Simplify unshare_files
    exec: Remove reset_files_struct
    kcmp: In kcmp_epoll_target use fget_task
    bpf: In bpf_task_fd_query use fget_task
    file: Implement fcheck_task
    proc/fd: In tid_fd_mode use fcheck_task
    proc/fd: In proc_fd_link use fcheck_task
    file: Implement fnext_task
    proc/fd: In proc_readfd_common use fnext_task
    bpf/task_iter: In task_file_seq_get_next use fnext_task
    proc/fd: In fdinfo seq_show don't use get_files_struct
    file: Remove get_files_struct
    file: Merge __fd_install into fd_install
    file: In f_dupfd read RLIMIT_NOFILE once.
    file: Merge __alloc_fd into alloc_fd
    file: Rename __close_fd to close_fd and remove the files parameter

    [1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
    Reported-by: Jeff Layton <jlayton@redhat.com>
    Reported-by: Daniel P. Berrangé <berrange@redhat.com>
    Suggested-by: Oleg Nesterov <oleg@redhat.com>
    Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

    Eric

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