whisper : guard null source in buffer loader read callback

whisper_init_from_buffer_with_params_no_state installs a read callback that
copies from buf->buffer + current_offset. When the buffer is exhausted (or the
supplied buffer is empty), size_to_copy is 0 and the source pointer can be null;
passing a null pointer to memcpy is undefined behavior even for a zero-length
copy (UBSan: 'null pointer passed as argument 2' at the memcpy). Loading a
crafted/short model through the buffer loader could hit this.

Skip the memcpy when there is nothing to copy. Loading from a null/empty or
truncated buffer now fails gracefully (returns NULL) with no UB.

This addresses bug 1 of #3879. Bug 2 (integer overflow when sizing the mel
filter buffer) is covered by the open PR #3780.

src/whisper.cpp

@@ -3695,7 +3695,12 @@ struct whisper_context * whisper_init_from_buffer_with_params_no_state(void * bu
 
         size_t size_to_copy = buf->current_offset + read_size < buf->size ? read_size : buf->size - buf->current_offset;
 
-        memcpy(output, buf->buffer + buf->current_offset, size_to_copy);
+        // When the buffer is exhausted size_to_copy is 0 and buf->buffer may be
+        // null (empty buffer); passing a null source to memcpy is undefined
+        // behavior even for a zero-length copy, so skip it (issue #3879).
+        if (size_to_copy > 0) {
+            memcpy(output, buf->buffer + buf->current_offset, size_to_copy);
+        }
         buf->current_offset += size_to_copy;
 
         return size_to_copy;

tests/CMakeLists.txt

@@ -96,6 +96,14 @@ target_link_libraries(${UTF8_TEST} PRIVATE common)
 add_test(NAME ${UTF8_TEST} COMMAND ${UTF8_TEST})
 set_tests_properties(${UTF8_TEST} PROPERTIES LABELS "unit")
 
+# Loading from a null/empty or truncated buffer must fail gracefully (#3879)
+set(BUFFER_LOADER_TEST test-whisper-buffer-loader)
+add_executable(${BUFFER_LOADER_TEST} ${BUFFER_LOADER_TEST}.cpp)
+target_include_directories(${BUFFER_LOADER_TEST} PRIVATE ../include ../ggml/include ../examples)
+target_link_libraries(${BUFFER_LOADER_TEST} PRIVATE common)
+add_test(NAME ${BUFFER_LOADER_TEST} COMMAND ${BUFFER_LOADER_TEST})
+set_tests_properties(${BUFFER_LOADER_TEST} PROPERTIES LABELS "unit;gh")
+
 # VAD test tests VAD in isolation
 set(VAD_TEST test-vad)
 add_executable(${VAD_TEST} ${VAD_TEST}.cpp)

tests/test-whisper-buffer-loader.cpp

@@ -0,0 +1,33 @@
+// Regression test for issue #3879 (bug 1):
+// The buffer-based model loader's read callback passed buf->buffer + offset to
+// memcpy even when nothing was left to copy. For an empty buffer that source is
+// NULL, and passing a NULL pointer to memcpy is undefined behavior even for a
+// zero-length copy. Loading from a null/empty or truncated buffer must fail
+// gracefully (return NULL) without invoking that UB.
+
+#include "whisper.h"
+
+#include <cstdint>
+#include <cstdio>
+
+#ifdef NDEBUG
+#undef NDEBUG
+#endif
+#include <cassert>
+
+int main() {
+    struct whisper_context_params cparams = whisper_context_default_params();
+    cparams.use_gpu = false;
+
+    // Empty buffer: the read callback is driven with a NULL source pointer.
+    struct whisper_context * ctx_empty = whisper_init_from_buffer_with_params(nullptr, 0, cparams);
+    assert(ctx_empty == nullptr);
+
+    // Truncated, non-model buffer: the loader runs out of bytes mid-read.
+    uint8_t truncated[8] = { 0 };
+    struct whisper_context * ctx_trunc = whisper_init_from_buffer_with_params(truncated, sizeof(truncated), cparams);
+    assert(ctx_trunc == nullptr);
+
+    printf("test-whisper-buffer-loader: OK\n");
+    return 0;
+}