gh-95782: Fix io.BufferedReader.tell() etc. being able to return offsets < 0 (GH-99709)

lseek() always returns 0 for character pseudo-devices like
`/dev/urandom` (for other non-regular files, e.g. `/dev/stdin`, it
always returns -1, to which CPython reacts by raising appropriate
exceptions). They are thus technically seekable despite not having seek
semantics.

When calling read() on e.g. an instance of `io.BufferedReader` that
wraps such a file, `BufferedReader` reads ahead, filling its buffer,
creating a discrepancy between the number of bytes read and the internal
`tell()` always returning 0, which previously resulted in e.g.
`BufferedReader.tell()` or `BufferedReader.seek()` being able to return
positions < 0 even though these are supposed to be always >= 0.

Invariably keep the return value non-negative by returning
max(former_return_value, 0) instead, and add some corresponding tests.

1 files changed, 10 insertions, 1 deletions

diff --git a/Modules/_io/bufferedio.c b/Modules/_io/bufferedio.c
index 8ebe9ec..b3450ee 100644
--- a/Modules/_io/bufferedio.c
+++ b/Modules/_io/bufferedio.c
@@ -1325,7 +1325,11 @@ _io__Buffered_tell_impl(buffered *self)
     if (pos == -1)
         return NULL;
     pos -= RAW_OFFSET(self);
-    /* TODO: sanity check (pos >= 0) */
+
+    // GH-95782
+    if (pos < 0)
+        pos = 0;
+
     return PyLong_FromOff_t(pos);
 }
 
@@ -1395,6 +1399,11 @@ _io__Buffered_seek_impl(buffered *self, PyObject *targetobj, int whence)
                 offset = target;
             if (offset >= -self->pos && offset <= avail) {
                 self->pos += offset;
+
+                // GH-95782
+                if (current - avail + offset < 0)
+                    return PyLong_FromOff_t(0);
+
                 return PyLong_FromOff_t(current - avail + offset);
             }
         }