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From 7819e9ce26a6331f7a347c59cebfd5c6a8902ea3 Mon Sep 17 00:00:00 2001
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From: Lukas Nykryn <lnykryn@redhat.com>
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Date: Thu, 15 Aug 2024 14:19:58 +0200
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Subject: [PATCH] =?UTF-8?q?Warn=20=E2=80=9Cfile=20changed=20as=20we=20read?=
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=?UTF-8?q?=20it=E2=80=9D=20less=20often?=
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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* src/create.c (dump_file0): Remove an fstatat call that is
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unnecessary because the file wasn’t read so we can treat the first
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fstatat as atomic. Warn “file changed” when the file’s size,
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mtime, user ID, group ID, or mode changes, instead of when the
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file’s size or ctime changes. Also, when such a change happens,
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do not change exit status if --ignore-failed-read. Finally, don’t
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attempt to change atime back if it didn’t change.
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---
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doc/tar.texi | 10 ++++++----
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src/create.c | 54 ++++++++++++++++++++++++++++++++++++----------------
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2 files changed, 44 insertions(+), 20 deletions(-)
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diff --git a/doc/tar.texi b/doc/tar.texi
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index b66b163..dd5a272 100644
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--- a/doc/tar.texi
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+++ b/doc/tar.texi
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@@ -2854,7 +2854,7 @@ Ignore exit codes of subprocesses. @xref{Writing to an External Program}.
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@opsummary{ignore-failed-read}
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@item --ignore-failed-read
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-Do not exit unsuccessfully merely because an unreadable file was encountered.
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+Do not exit unsuccessfully merely because reading failed.
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@xref{Ignore Failed Read}.
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@opsummary{ignore-zeros}
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@@ -4638,7 +4638,8 @@ Disable all warning messages.
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@item file-changed
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@samp{%s: file changed as we read it}
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@item failed-read
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-Suppresses warnings about unreadable files or directories. This
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+Suppresses warnings about read failures, which can occur if files
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+or directories are unreadable, or if they change while being read. This
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keyword applies only if used together with the @option{--ignore-failed-read}
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option. @xref{Ignore Failed Read}.
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@end table
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@@ -5761,11 +5762,12 @@ Disable SELinux context support.
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@table @option
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@item --ignore-failed-read
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@opindex ignore-failed-read
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-Do not exit with nonzero on unreadable files or directories.
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+Do not exit with nonzero if there are mild problems while reading.
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@end table
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This option has effect only during creation. It instructs tar to
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-treat as mild conditions any missing or unreadable files (directories).
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+treat as mild conditions any missing or unreadable files (directories),
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+or files that change while reading.
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Such failures don't affect the program exit code, and the
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corresponding diagnostic messages are marked as warnings, not errors.
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These warnings can be suppressed using the
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diff --git a/src/create.c b/src/create.c
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index e2816fc..2b3001d 100644
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--- a/src/create.c
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+++ b/src/create.c
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@@ -1650,8 +1650,6 @@ dump_file0 (struct tar_stat_info *st, char const *name, char const *p)
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{
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union block *header;
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char type;
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- off_t original_size;
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- struct timespec original_ctime;
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off_t block_ordinal = -1;
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int fd = 0;
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bool is_dir;
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@@ -1694,10 +1692,11 @@ dump_file0 (struct tar_stat_info *st, char const *name, char const *p)
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return;
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}
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- st->archive_file_size = original_size = st->stat.st_size;
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+ struct stat st1 = st->stat;
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+ st->archive_file_size = st->stat.st_size;
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st->atime = get_stat_atime (&st->stat);
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st->mtime = get_stat_mtime (&st->stat);
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- st->ctime = original_ctime = get_stat_ctime (&st->stat);
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+ st->ctime = get_stat_ctime (&st->stat);
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#ifdef S_ISHIDDEN
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if (S_ISHIDDEN (st->stat.st_mode))
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@@ -1747,7 +1746,7 @@ dump_file0 (struct tar_stat_info *st, char const *name, char const *p)
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if (is_dir || S_ISREG (st->stat.st_mode) || S_ISCTG (st->stat.st_mode))
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{
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bool ok;
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- struct stat final_stat;
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+ struct stat st2;
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xattrs_acls_get (parentfd, name, st, 0, !is_dir);
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xattrs_selinux_get (parentfd, name, st, fd);
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@@ -1815,31 +1814,54 @@ dump_file0 (struct tar_stat_info *st, char const *name, char const *p)
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errno = - parentfd;
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ok = false;
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}
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- else
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- ok = fstatat (parentfd, name, &final_stat, fstatat_flags) == 0;
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}
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else
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- ok = fstat (fd, &final_stat) == 0;
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+ ok = fstat (fd, &st2) == 0;
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if (! ok)
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file_removed_diag (p, top_level, stat_diag);
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}
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- if (ok)
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+ if (ok && fd)
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{
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- if ((timespec_cmp (get_stat_ctime (&final_stat), original_ctime) != 0
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- /* Original ctime will change if the file is a directory and
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- --remove-files is given */
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- && !(remove_files_option && is_dir))
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- || original_size < final_stat.st_size)
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+ /* Heuristically check whether the file is the same in all
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+ attributes that tar cares about and can easily check.
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+ Although the check is not perfect since it does not
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+ consult file contents, it is typically good enough.
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+ Do not check atime which is saved only to replace it later.
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+ Do not check ctime where changes might be benign (e.g.,
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+ another process creates a hard link to the file). */
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+
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+ /* If the file's user ID, group ID or mode changed, tar may
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+ have output the wrong info for the file. */
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+ ok &= st1.st_uid == st2.st_uid;
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+ ok &= st1.st_gid == st2.st_gid;
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+ ok &= st1.st_mode == st2.st_mode;
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+
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+ /* Likewise for the file's mtime, but skip this check if it
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+ is a directory possibly updated by --remove-files. */
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+ if (! (is_dir && remove_files_option))
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+ ok &= ! timespec_cmp (get_stat_mtime (&st1),
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+ get_stat_mtime (&st2));
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+
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+ /* Likewise for the file's size, but skip this check if it
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+ is a directory as tar does not output directory sizes.
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+ Although dump_regular_file caught regular file shrinkage,
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+ it shouldn't hurt to check for shrinkage again now;
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+ plus, the file may have grown. */
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+ if (!is_dir)
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+ ok &= st1.st_size == st2.st_size;
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+
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+ if (!ok)
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{
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WARNOPT (WARN_FILE_CHANGED,
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(0, 0, _("%s: file changed as we read it"),
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quotearg_colon (p)));
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- set_exit_status (TAREXIT_DIFFERS);
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+ if (! ignore_failed_read_option)
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+ set_exit_status (TAREXIT_DIFFERS);
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}
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else if (atime_preserve_option == replace_atime_preserve
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- && fd && (is_dir || original_size != 0)
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+ && timespec_cmp (st->atime, get_stat_atime (&st2)) != 0
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&& set_file_atime (fd, parentfd, name, st->atime) != 0
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&& errno != EROFS )
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utime_error (p);
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--
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2.45.2
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