s390/kexec_file: print some more error messages

Be kind and give some more information on what went wrong.
Signed-off-by: Philipp Rudo <prudo@redhat.com>
Link: https://lore.kernel.org/r/20211208130741.5821-2-prudo@redhat.comSigned-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/machine_kexec_file.c

@@ -7,6 +7,8 @@
  * Author(s): Philipp Rudo <prudo@linux.vnet.ibm.com>
  */
 
+#define pr_fmt(fmt)	"kexec: " fmt
+
 #include <linux/elf.h>
 #include <linux/errno.h>
 #include <linux/kexec.h>
@@ -290,9 +292,16 @@ int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
 				     const Elf_Shdr *relsec,
 				     const Elf_Shdr *symtab)
 {
+	const char *strtab, *name, *shstrtab;
+	const Elf_Shdr *sechdrs;
 	Elf_Rela *relas;
 	int i, r_type;
 
+	/* String & section header string table */
+	sechdrs = (void *)pi->ehdr + pi->ehdr->e_shoff;
+	strtab = (char *)pi->ehdr + sechdrs[symtab->sh_link].sh_offset;
+	shstrtab = (char *)pi->ehdr + sechdrs[pi->ehdr->e_shstrndx].sh_offset;
+
 	relas = (void *)pi->ehdr + relsec->sh_offset;
 
 	for (i = 0; i < relsec->sh_size / sizeof(*relas); i++) {
@@ -304,15 +313,27 @@ int arch_kexec_apply_relocations_add(struct purgatory_info *pi,
 		sym = (void *)pi->ehdr + symtab->sh_offset;
 		sym += ELF64_R_SYM(relas[i].r_info);
 
-		if (sym->st_shndx == SHN_UNDEF)
+		if (sym->st_name)
+			name = strtab + sym->st_name;
+		else
+			name = shstrtab + sechdrs[sym->st_shndx].sh_name;
+
+		if (sym->st_shndx == SHN_UNDEF) {
+			pr_err("Undefined symbol: %s\n", name);
 			return -ENOEXEC;
+		}
 
-		if (sym->st_shndx == SHN_COMMON)
+		if (sym->st_shndx == SHN_COMMON) {
+			pr_err("symbol '%s' in common section\n", name);
 			return -ENOEXEC;
+		}
 
 		if (sym->st_shndx >= pi->ehdr->e_shnum &&
-		    sym->st_shndx != SHN_ABS)
+		    sym->st_shndx != SHN_ABS) {
+			pr_err("Invalid section %d for symbol %s\n",
+			       sym->st_shndx, name);
 			return -ENOEXEC;
+		}
 
 		loc = pi->purgatory_buf;
 		loc += section->sh_offset;