From 7b47b3dd214c8ff2c699f13efe5533941be53635 Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Tue, 8 Apr 2025 18:38:38 +0200
Subject: [PATCH] libio: Synthesize ESPIPE error if lseek returns 0 after
 reading bytes
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This is required so that fclose, when trying to seek to the right
position after filling the input buffer, does not fail with EINVAL.
This fclose code path only ignores ESPIPE errors.

Reported by Petr Pisar on
<https://bugzilla.redhat.com/show_bug.cgi?id=2358265>.

Fixes commit be6818be31e756398e45f70e2819d78be0961223 ("Make fclose
seek input file to right offset (bug 12724)").

Reviewed-by: Frédéric Bérat <fberat@redhat.com>
---
 libio/fileops.c                   | 10 +++++++
 stdio-common/Makefile             |  1 +
 stdio-common/tst-fclose-devzero.c | 50 +++++++++++++++++++++++++++++++
 3 files changed, 61 insertions(+)
 create mode 100644 stdio-common/tst-fclose-devzero.c

diff --git a/libio/fileops.c b/libio/fileops.c
index a59e248142..0cce828859 100644
--- a/libio/fileops.c
+++ b/libio/fileops.c
@@ -928,6 +928,16 @@ do_ftell (FILE *fp)
   if (result == EOF)
     return result;
 
+  if (result == 0 && offset < 0)
+    {
+      /* This happens for some character devices that always report
+	 file offset 0 even after some data has been read (instead of
+	 failing with ESPIPE).  The fclose path ignores this
+	 error.  */
+      __set_errno (ESPIPE);
+      return EOF;
+    }
+
   result += offset;
 
   if (result < 0)
diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index 3709222266..64b3575acb 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -262,6 +262,7 @@ tests := \
   tst-bz11319-fortify2 \
   tst-cookie \
   tst-dprintf-length \
+  tst-fclose-devzero \
   tst-fclose-offset \
   tst-fdopen \
   tst-fdopen2 \
diff --git a/stdio-common/tst-fclose-devzero.c b/stdio-common/tst-fclose-devzero.c
new file mode 100644
index 0000000000..1c7b39a3e0
--- /dev/null
+++ b/stdio-common/tst-fclose-devzero.c
@@ -0,0 +1,50 @@
+/* Test that always-zero lseek does not cause fclose failure after fread.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
+
+#include <support/check.h>
+#include <support/xstdio.h>
+
+int
+do_test (void)
+{
+  for (int do_ftello = 0; do_ftello < 2; ++do_ftello)
+    {
+      FILE *fp = xfopen ("/dev/zero", "r");
+      char buf[17];
+      memset (buf, 0xcc, sizeof (buf));
+      xfread (buf, 1, sizeof (buf), fp);
+      static const char zeros[sizeof (buf)] = { 0 };
+      TEST_COMPARE_BLOB (buf, sizeof (buf), zeros, sizeof (zeros));
+      if (do_ftello)
+        {
+          errno = 0;
+          TEST_COMPARE (ftello (fp), -1);
+          TEST_COMPARE (errno, ESPIPE);
+        }
+      /* Do not use xfclose because it flushes first.  */
+      TEST_COMPARE (fclose (fp), 0);
+    }
+
+  return 0;
+}
+
+#include <support/test-driver.c>