libdwfl: Add some extra space to buffer to read kernel image header

GCC 14 notices we play some tricks with the array into which we try
to read the kernel image header.

image-header.c: In function ‘__libdw_image_header’:
image-header.c:77:18: error: array subscript -496 is outside array bounds of ‘char[96]’ [-Werror=array-bounds=]
   77 |           header = header_buffer - H_START;
      |                  ^
image-header.c:67:12: note: at offset -496 into object ‘header_buffer’ of size 96
   67 |       char header_buffer[H_READ_SIZE];
      |            ^~~~~~~~~~~~~

GCC is correct. The new header pointer is before the actually buffer we
want to read from. Later in the code we "correct" the address again by
adding the "offset" off the elements we want to read. Such pointer
arithmetic is technically invalid. Make it valid by making the buffer
a little bigger, so all pointer arithmetic stays inside the header_buffer.
This does waste 496 bytes on the stack at the front of the buffer that
is never used.

	* libdwfl/image-header.c (__libdw_image_header): Add H_START
	to header_buffer size and return

Signed-off-by: Mark Wielaard <mark@klomp.org>

diff --git a/libdwfl/image-header.c b/libdwfl/image-header.c
index c777cc84959938c8e4d1d23cb0138745af06b4fc..03cd7a7616200d66ad8ec8145c1c8e91c56a6bac 100644 (file)
--- a/libdwfl/image-header.c
+++ b/libdwfl/image-header.c
@@ -1,6 +1,6 @@
 /* Linux kernel image support for libdwfl.
    Copyright (C) 2009-2011 Red Hat, Inc.
-   Copyright (C) 2022 Mark J. Wielaard <mark@klomp.org>
+   Copyright (C) 2022, 2024 Mark J. Wielaard <mark@klomp.org>
    This file is part of elfutils.
 
    This file is free software; you can redistribute it and/or modify
@@ -64,17 +64,17 @@ __libdw_image_header (int fd, off_t *start_offset,
   if (likely (mapped_size > H_END))
     {
       const void *header = mapped;
-      char header_buffer[H_READ_SIZE];
+      char header_buffer[H_READ_SIZE + H_START];
       if (header == NULL)
        {
-         ssize_t n = pread_retry (fd, header_buffer, H_READ_SIZE,
+         ssize_t n = pread_retry (fd, header_buffer + H_START, H_READ_SIZE,
                                   *start_offset + H_START);
          if (n < 0)
            return DWFL_E_ERRNO;
          if (n < H_READ_SIZE)
            return DWFL_E_BADELF;
 
-         header = header_buffer - H_START;
+         header = header_buffer;
        }
 
       uint16_t magic1;