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gfileutils: Fix potential integer overflow in g_get_current_dir()

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In practice, this will never happen.

If `getcwd()` returns `ERANGE` whenever the working directory is ≥
`PATH_MAX`, though, the previous implementation of the loop would run
until `max_len == G_MAXULONG`, and would then overflow when adding `1`
to it for a nul terminator in the allocation.

Avoid that problem by always keeping `buffer_len` as a power of two, and
relying on `getcwd()` to write a nul terminator within the buffer space
it’s given. It seems to do this on all platforms we care about, because
the previous version of the code never explicitly wrote a nul terminator
anywhere.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #98
This commit is contained in:
Philip Withnall 2023-04-13 17:03:35 +01:00
parent 9e83c0e37e
commit 7a5d4c2bb7
1 changed files with 14 additions and 10 deletions
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24
glib/gfileutils.c
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@ -2940,7 +2940,7 @@ g_get_current_dir (void)
const gchar *pwd; const gchar *pwd;
gchar *buffer = NULL; gchar *buffer = NULL;
gchar *dir = NULL; gchar *dir = NULL;
static gulong max_len = 0; static gsize buffer_size = 0;
struct stat pwdbuf, dotbuf; struct stat pwdbuf, dotbuf;
pwd = g_getenv ("PWD"); pwd = g_getenv ("PWD");
@ -2949,27 +2949,31 @@ g_get_current_dir (void)
dotbuf.st_dev == pwdbuf.st_dev && dotbuf.st_ino == pwdbuf.st_ino) dotbuf.st_dev == pwdbuf.st_dev && dotbuf.st_ino == pwdbuf.st_ino)
return g_strdup (pwd); return g_strdup (pwd);
if (max_len == 0) if (buffer_size == 0)
max_len = (G_PATH_LENGTH == -1) ? 2048 : G_PATH_LENGTH; buffer_size = (G_PATH_LENGTH == -1) ? 2048 : G_PATH_LENGTH;
while (max_len < G_MAXULONG / 2) while (buffer_size < G_MAXSIZE / 2)
{ {
g_free (buffer); g_free (buffer);
buffer = g_new (gchar, max_len + 1); buffer = g_new (gchar, buffer_size);
*buffer = 0; *buffer = 0;
dir = getcwd (buffer, max_len); dir = getcwd (buffer, buffer_size);
if (dir || errno != ERANGE) if (dir || errno != ERANGE)
break; break;
max_len *= 2; buffer_size *= 2;
} }
/* Check that getcwd() nul-terminated the string. It should do, but the specs
* don’t actually explicitly state that:
* https://pubs.opengroup.org/onlinepubs/9699919799/functions/getcwd.html */
g_assert (dir == NULL || strnlen (dir, buffer_size) < buffer_size);
if (!dir || !*buffer) if (!dir || !*buffer)
{ {
/* hm, should we g_error() out here? /* Fallback return value */
* this can happen if e.g. "./" has mode \0000 g_assert (buffer_size >= 2);
*/
buffer[0] = G_DIR_SEPARATOR; buffer[0] = G_DIR_SEPARATOR;
buffer[1] = 0; buffer[1] = 0;
} }