From 73d119f4f8052a9fb6cef13cd9e75d5a4e23311a Mon Sep 17 00:00:00 2001

From: dann frazier <dann.frazier@canonical.com>

Date: Wed, 29 Nov 2023 18:32:34 -0700

Subject: [PATCH] tail: fix tailing sysfs files where PAGE_SIZE > BUFSIZ

* src/tail.c (file_lines): Ensure we use a buffer size >= PAGE_SIZE when

searching backwards to avoid seeking within a file,

which on sysfs files is accepted but also returns no data.

* tests/tail/tail-sysfs.sh: Add a new test.

* tests/local.mk: Reference the new test.

* NEWS: Mention the bug fix.

Fixes https://bugs.gnu.org/67490

Upstream-commit: 73d119f4f8052a9fb6cef13cd9e75d5a4e23311a

Cherry-picked-by: Lukáš Zaoral <lzaoral@redhat.com>

---

 src/tail.c               | 57 +++++++++++++++++++++++++++++-----------

 tests/local.mk           |  1 +

 tests/tail/tail-sysfs.sh | 34 ++++++++++++++++++++++++

 3 files changed, 77 insertions(+), 15 deletions(-)

 create mode 100755 tests/tail/tail-sysfs.sh

diff --git a/src/tail.c b/src/tail.c

index c45f3b65a..6979e0ba3 100644

--- a/src/tail.c

+++ b/src/tail.c

@@ -208,6 +208,9 @@ static int nbpids = 0;

    that is writing to all followed files.  */

 static pid_t pid;

+/* Used to determine the buffer size when scanning backwards in a file.  */

+static idx_t page_size;

+

 /* True if we have ever read standard input.  */

 static bool have_read_stdin;

@@ -515,22 +518,40 @@ xlseek (int fd, off_t offset, int whence, char const *filename)

    Return true if successful.  */

 static bool

-file_lines (char const *pretty_filename, int fd, uintmax_t n_lines,

-            off_t start_pos, off_t end_pos, uintmax_t *read_pos)

+file_lines (char const *pretty_filename, int fd, struct stat const *sb,

+            uintmax_t n_lines, off_t start_pos, off_t end_pos,

+            uintmax_t *read_pos)

 {

-  char buffer[BUFSIZ];

+  char *buffer;

   size_t bytes_read;

+  blksize_t bufsize = BUFSIZ;

   off_t pos = end_pos;

+  bool ok = true;

   if (n_lines == 0)

     return true;

+  /* Be careful with files with sizes that are a multiple of the page size,

+     as on /proc or /sys file systems these files accept seeking to within

+     the file, but then return no data when read.  So use a buffer that's

+     at least PAGE_SIZE to avoid seeking within such files.

+

+     We could also indirectly use a large enough buffer through io_blksize()

+     however this would be less efficient in the common case, as it would

+     generally pick a larger buffer size, resulting in reading more data

+     from the end of the file.  */

+  affirm (S_ISREG (sb->st_mode));

+  if (sb->st_size % page_size == 0)

+    bufsize = MAX (BUFSIZ, page_size);

+

+  buffer = xmalloc (bufsize);

+

   /* Set 'bytes_read' to the size of the last, probably partial, buffer;

-     0 < 'bytes_read' <= 'BUFSIZ'.  */

-  bytes_read = (pos - start_pos) % BUFSIZ;

+     0 < 'bytes_read' <= 'bufsize'.  */

+  bytes_read = (pos - start_pos) % bufsize;

   if (bytes_read == 0)

-    bytes_read = BUFSIZ;

-  /* Make 'pos' a multiple of 'BUFSIZ' (0 if the file is short), so that all

+    bytes_read = bufsize;

+  /* Make 'pos' a multiple of 'bufsize' (0 if the file is short), so that all

      reads will be on block boundaries, which might increase efficiency.  */

   pos -= bytes_read;

   xlseek (fd, pos, SEEK_SET, pretty_filename);

@@ -538,7 +559,8 @@ file_lines (char const *pretty_filename, int fd, uintmax_t n_lines,

   if (bytes_read == SAFE_READ_ERROR)

     {

       error (0, errno, _("error reading %s"), quoteaf (pretty_filename));

-      return false;

+      ok = false;

+      goto free_buffer;

     }

   *read_pos = pos + bytes_read;

@@ -565,7 +587,7 @@ file_lines (char const *pretty_filename, int fd, uintmax_t n_lines,

               xwrite_stdout (nl + 1, bytes_read - (n + 1));

               *read_pos += dump_remainder (false, pretty_filename, fd,

                                            end_pos - (pos + bytes_read));

-              return true;

+              goto free_buffer;

             }

         }

@@ -577,23 +599,26 @@ file_lines (char const *pretty_filename, int fd, uintmax_t n_lines,

           xlseek (fd, start_pos, SEEK_SET, pretty_filename);

           *read_pos = start_pos + dump_remainder (false, pretty_filename, fd,

                                                   end_pos);

-          return true;

+          goto free_buffer;

         }

-      pos -= BUFSIZ;

+      pos -= bufsize;

       xlseek (fd, pos, SEEK_SET, pretty_filename);

-      bytes_read = safe_read (fd, buffer, BUFSIZ);

+      bytes_read = safe_read (fd, buffer, bufsize);

       if (bytes_read == SAFE_READ_ERROR)

         {

           error (0, errno, _("error reading %s"), quoteaf (pretty_filename));

-          return false;

+          ok = false;

+          goto free_buffer;

         }

       *read_pos = pos + bytes_read;

     }

   while (bytes_read > 0);

-  return true;

+free_buffer:

+  free (buffer);

+  return ok;

 }

 /* Print the last N_LINES lines from the end of the standard input,

@@ -1915,7 +1940,7 @@ tail_lines (char const *pretty_filename, int fd, uintmax_t n_lines,

         {

           *read_pos = end_pos;

           if (end_pos != 0

-              && ! file_lines (pretty_filename, fd, n_lines,

+              && ! file_lines (pretty_filename, fd, &stats, n_lines,

                                start_pos, end_pos, read_pos))

             return false;

         }

@@ -2337,6 +2362,8 @@ main (int argc, char **argv)

   atexit (close_stdout);

+  page_size = getpagesize ();

+

   have_read_stdin = false;

   count_lines = true;

diff --git a/tests/local.mk b/tests/local.mk

index db4ee7ed8..bf03238c3 100644

--- a/tests/local.mk

+++ b/tests/local.mk

@@ -257,6 +257,7 @@ all_tests =					\

   tests/seq/seq-precision.sh			\

   tests/head/head.pl				\

   tests/head/head-elide-tail.pl			\

+  tests/tail/tail-sysfs.sh			\

   tests/tail/tail-n0f.sh			\

   tests/ls/ls-misc.pl				\

   tests/date/date.pl				\

diff --git a/tests/tail/tail-sysfs.sh b/tests/tail/tail-sysfs.sh

new file mode 100755

index 000000000..00874b3dc

--- /dev/null

+++ b/tests/tail/tail-sysfs.sh

@@ -0,0 +1,34 @@

+#!/bin/sh

+# sysfs files have weird properties that can be influenced by page size

+

+# Copyright 2023 Free Software Foundation, Inc.

+

+# This program is free software: you can redistribute it and/or modify

+# it under the terms of the GNU General Public License as published by

+# the Free Software Foundation, either version 3 of the License, or

+# (at your option) any later version.

+

+# This program is distributed in the hope that it will be useful,

+# but WITHOUT ANY WARRANTY; without even the implied warranty of

+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

+# GNU General Public License for more details.

+

+# You should have received a copy of the GNU General Public License

+# along with this program.  If not, see <https://www.gnu.org/licenses/>.

+

+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src

+print_ver_ tail

+

+file='/sys/kernel/profiling'

+

+test -r "$file" || skip_ "No $file file"

+

+cp -f "$file" exp || framework_failure_

+

+tail -n1 "$file" > out || fail=1

+compare exp out || fail=1

+

+tail -c2 "$file" > out || fail=1

+compare exp out || fail=1

+

+Exit $fail