bpo-29649: Improve struct.pack_into() boundary error messages (#424)
diff --git a/Modules/_struct.c b/Modules/_struct.c
index f66ee18..4bc4186 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1931,14 +1931,40 @@
}
/* Support negative offsets. */
- if (offset < 0)
+ if (offset < 0) {
+ /* Check that negative offset is low enough to fit data */
+ if (offset + soself->s_size > 0) {
+ PyErr_Format(StructError,
+ "no space to pack %zd bytes at offset %zd",
+ soself->s_size,
+ offset);
+ PyBuffer_Release(&buffer);
+ return NULL;
+ }
+
+ /* Check that negative offset is not crossing buffer boundary */
+ if (offset + buffer.len < 0) {
+ PyErr_Format(StructError,
+ "offset %zd out of range for %zd-byte buffer",
+ offset,
+ buffer.len);
+ PyBuffer_Release(&buffer);
+ return NULL;
+ }
+
offset += buffer.len;
+ }
/* Check boundaries */
- if (offset < 0 || (buffer.len - offset) < soself->s_size) {
+ if ((buffer.len - offset) < soself->s_size) {
PyErr_Format(StructError,
- "pack_into requires a buffer of at least %zd bytes",
- soself->s_size);
+ "pack_into requires a buffer of at least %zd bytes for "
+ "packing %zd bytes at offset %zd "
+ "(actual buffer size is %zd)",
+ soself->s_size + offset,
+ soself->s_size,
+ offset,
+ buffer.len);
PyBuffer_Release(&buffer);
return NULL;
}