kernel/kexec_file.c: make purgatory_info->ehdr const

The kexec_purgatory buffer is read-only.  Thus all pointers into
kexec_purgatory are read-only, too.  Point this out by explicitly
marking purgatory_info->ehdr as 'const' and update the comments in
purgatory_info.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Philipp Rudo <[email protected]>
Acked-by: Dave Young <[email protected]>
Cc: AKASHI Takahiro <[email protected]>
Cc: Eric Biederman <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Thiago Jung Bauermann <[email protected]>
Cc: Vivek Goyal <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/kexec.h

@@ -99,14 +99,19 @@ struct compat_kexec_segment {
 
 #ifdef CONFIG_KEXEC_FILE
 struct purgatory_info {
-	/* Pointer to elf header of read only purgatory */
-	Elf_Ehdr *ehdr;
-
-	/* Pointer to purgatory sechdrs which are modifiable */
+	/*
+	 * Pointer to elf header at the beginning of kexec_purgatory.
+	 * Note: kexec_purgatory is read only
+	 */
+	const Elf_Ehdr *ehdr;
+	/*
+	 * Temporary, modifiable buffer for sechdrs used for relocation.
+	 * This memory can be freed post image load.
+	 */
 	Elf_Shdr *sechdrs;
 	/*
-	 * Temporary buffer location where purgatory is loaded and relocated
-	 * This memory can be freed post image load
+	 * Temporary, modifiable buffer for stripped purgatory used for
+	 * relocation. This memory can be freed post image load.
 	 */
 	void *purgatory_buf;
 

kernel/kexec_file.c

@@ -941,7 +941,7 @@ int kexec_load_purgatory(struct kimage *image, unsigned long min,
 	if (kexec_purgatory_size <= 0)
 		return -EINVAL;
 
-	pi->ehdr = (Elf_Ehdr *)kexec_purgatory;
+	pi->ehdr = (const Elf_Ehdr *)kexec_purgatory;
 
 	ret = __kexec_load_purgatory(image, min, max, top_down);
 	if (ret)
@@ -965,9 +965,9 @@ int kexec_load_purgatory(struct kimage *image, unsigned long min,
 static Elf_Sym *kexec_purgatory_find_symbol(struct purgatory_info *pi,
 					    const char *name)
 {
+	const Elf_Ehdr *ehdr;
 	Elf_Sym *syms;
 	Elf_Shdr *sechdrs;
-	Elf_Ehdr *ehdr;
 	int i, k;
 	const char *strtab;