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Clarify g_file_test() docs about TOCTOU bugs

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Do not show just what not to do: show what to do instead, otherwise
people won't know how to fix their code.

Make sure to link to an explanation of the TOCTOU class of bugs;
Wikipedia is as good a place as any.
This commit is contained in:
Emmanuele Bassi 2023-01-26 14:20:34 +00:00
parent b5a3297dca
commit 293b492334
1 changed files with 19 additions and 1 deletions
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20
glib/gfileutils.c
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@ -313,10 +313,13 @@ g_mkdir_with_parents (const gchar *pathname,
* *
* You should never use g_file_test() to test whether it is safe * You should never use g_file_test() to test whether it is safe
* to perform an operation, because there is always the possibility * to perform an operation, because there is always the possibility
* of the condition changing before you actually perform the operation. * of the condition changing before you actually perform the operation,
* see [TOCTOU](https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use).
*
* For example, you might think you could use %G_FILE_TEST_IS_SYMLINK * For example, you might think you could use %G_FILE_TEST_IS_SYMLINK
* to know whether it is safe to write to a file without being * to know whether it is safe to write to a file without being
* tricked into writing into a different location. It doesn't work! * tricked into writing into a different location. It doesn't work!
*
* |[<!-- language="C" --> * |[<!-- language="C" -->
* // DON'T DO THIS * // DON'T DO THIS
* if (!g_file_test (filename, G_FILE_TEST_IS_SYMLINK)) * if (!g_file_test (filename, G_FILE_TEST_IS_SYMLINK))
@ -324,6 +327,21 @@ g_mkdir_with_parents (const gchar *pathname,
* fd = g_open (filename, O_WRONLY); * fd = g_open (filename, O_WRONLY);
* // write to fd * // write to fd
* } * }
*
* // DO THIS INSTEAD
* fd = g_open (filename, O_WRONLY);
* if (fd == -1)
* {
* // check error
* if (errno == ELOOP)
* // file is a symlink and can be ignored
* else
* // handle errors as before
* }
* else
* {
* // write to fd
* }
* ]| * ]|
* *
* Another thing to note is that %G_FILE_TEST_EXISTS and * Another thing to note is that %G_FILE_TEST_EXISTS and