[Bug nscd/23178] sudo will fail when it is run in concurrent with commands that changes /etc/passwd

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=23178

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=8c78faa9ef5c6cae455739f162e4b9d690e32eca

commit 8c78faa9ef5c6cae455739f162e4b9d690e32eca
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Wed May 16 10:51:15 2018 -0300

    Fix concurrent changes on nscd aware files (BZ #23178)

    As indicated by BZ#23178, concurrent access on some files read by nscd
    may result non expected data send through service requisition.  This is
    due 'sendfile' Linux implementation where for sockets with zero-copy
    support, callers must ensure the transferred portions of the the file
    reffered by input file descriptor remain unmodified until the reader
    on the other end of socket has consumed the transferred data.

    I could not find any explicit documentation stating this behaviour on
    Linux kernel documentation.  However man-pages sendfile entry [1] states
    in NOTES the aforementioned remark.  It was initially pushed on man-pages
    with an explicit testcase [2] that shows changing the file used in
    'sendfile' call prior the socket input data consumption results in
    previous data being lost.

    From commit message it stated on tested Linux version (3.15) only TCP
    socket showed this issues, however on recent kernels (4.4) I noticed the
    same behaviour for local sockets as well.

    Since sendfile on HURD is a read/write operation and the underlying
    issue on Linux, the straightforward fix is just remove sendfile use
    altogether.  I am really skeptical it is hitting some hotstop (there
    are indication over internet that sendfile is helpfull only for large
    files, more than 10kb) here to justify that extra code complexity or
    to pursuit other possible fix (through memory or file locks for
    instance, which I am not sure it is doable).

    Checked on x86_64-linux-gnu.

        [BZ #23178]
        * nscd/nscd-client.h (sendfileall): Remove prototype.
        * nscd/connections.c [HAVE_SENDFILE] (sendfileall): Remove function.
        (handle_request): Use writeall instead of sendfileall.
        * nscd/aicache.c (addhstaiX): Likewise.
        * nscd/grpcache.c (cache_addgr): Likewise.
        * nscd/hstcache.c (cache_addhst): Likewise.
        * nscd/initgrcache.c (addinitgroupsX): Likewise.
        * nscd/netgroupcache.c (addgetnetgrentX, addinnetgrX): Likewise.
        * nscd/pwdcache.c (cache_addpw): Likewise.
        * nscd/servicescache.c (cache_addserv): Likewise.
        * sysdeps/unix/sysv/linux/Makefile [$(subdir) == nscd]
        (sysdep-CFLAGS): Remove -DHAVE_SENDFILE.
        * sysdeps/unix/sysv/linux/kernel-features.h (__ASSUME_SENDFILE):
        Remove define.

    [1] http://man7.org/linux/man-pages/man2/sendfile.2.html
    [2]
https://github.com/mkerrisk/man-pages/commit/7b6a3299776b5c1c4f169a591434a855d50c68b4#diff-efd6af3a70f0f07c578e85b51e83b3c3

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                                 |   18 +++++++++
 nscd/aicache.c                            |   30 +--------------
 nscd/connections.c                        |   54 +-------------------------
 nscd/grpcache.c                           |   37 +----------------
 nscd/hstcache.c                           |   37 +----------------
 nscd/initgrcache.c                        |   37 +----------------
 nscd/netgroupcache.c                      |   59 +---------------------------
 nscd/nscd-client.h                        |    2 -
 nscd/pwdcache.c                           |   37 +----------------
 nscd/servicescache.c                      |   34 +---------------
 sysdeps/unix/sysv/linux/Makefile          |    2 +-
 sysdeps/unix/sysv/linux/kernel-features.h |    3 -
 12 files changed, 40 insertions(+), 310 deletions(-)

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