x86/boot: Robustify calling startup_{32,64}() from the decompressor code

After commit ce697cc ("kbuild: remove head-y syntax"), I
started digging whether x86 is ready for removing this old cruft.
Removing its objects from the list makes the kernel unbootable.
This applies only to bzImage, vmlinux still works correctly.
The reason is that with no strict object order determined by the
linker arguments, not the linker script, startup_64 can be placed
not right at the beginning of the kernel.
Here's vmlinux.map's beginning before removing:

  ffffffff81000000         vmlinux.o:(.head.text)
  ffffffff81000000                 startup_64
  ffffffff81000070                 secondary_startup_64
  ffffffff81000075                 secondary_startup_64_no_verify
  ffffffff81000160                 verify_cpu

and after:

  ffffffff81000000         vmlinux.o:(.head.text)
  ffffffff81000000                 pvh_start_xen
  ffffffff81000080                 startup_64
  ffffffff810000f0                 secondary_startup_64
  ffffffff810000f5                 secondary_startup_64_no_verify

Not a problem itself, but the self-extractor code has the address of
that function hardcoded the beginning, not looking onto the ELF
header, which always contains the address of startup_{32,64}().

So, instead of doing an "act of blind faith", just take the address
from the ELF header and extract a relative offset to the entry
point. The decompressor function already returns a pointer to the
beginning of the kernel to the Asm code, which then jumps to it,
so add that offset to the return value.
This doesn't change anything for now, but allows to resign from the
"head object list" for x86 and makes sure valid Kbuild or any other
improvements won't break anything here in general.

Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Jiri Slaby <jirislaby@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20230109170403.4117105-2-alexandr.lobakin@intel.com

arch/x86/boot/compressed/head_32.S

@@ -187,7 +187,7 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
 	leal	boot_heap@GOTOFF(%ebx), %eax
 	pushl	%eax			/* heap area */
 	pushl	%esi			/* real mode pointer */
-	call	extract_kernel		/* returns kernel location in %eax */
+	call	extract_kernel		/* returns kernel entry point in %eax */
 	addl	$24, %esp
 
 /*

arch/x86/boot/compressed/head_64.S

@@ -569,7 +569,7 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
 	movl	input_len(%rip), %ecx	/* input_len */
 	movq	%rbp, %r8		/* output target address */
 	movl	output_len(%rip), %r9d	/* decompressed length, end of relocs */
-	call	extract_kernel		/* returns kernel location in %rax */
+	call	extract_kernel		/* returns kernel entry point in %rax */
 	popq	%rsi
 
 /*

arch/x86/boot/compressed/misc.c

@@ -277,7 +277,7 @@ static inline void handle_relocations(void *output, unsigned long output_len,
 { }
 #endif
 
-static void parse_elf(void *output)
+static size_t parse_elf(void *output)
 {
 #ifdef CONFIG_X86_64
 	Elf64_Ehdr ehdr;
@@ -293,10 +293,8 @@ static void parse_elf(void *output)
 	if (ehdr.e_ident[EI_MAG0] != ELFMAG0 ||
 	   ehdr.e_ident[EI_MAG1] != ELFMAG1 ||
 	   ehdr.e_ident[EI_MAG2] != ELFMAG2 ||
-	   ehdr.e_ident[EI_MAG3] != ELFMAG3) {
+	   ehdr.e_ident[EI_MAG3] != ELFMAG3)
 		error("Kernel is not a valid ELF file");
-		return;
-	}
 
 	debug_putstr("Parsing ELF... ");
 
@@ -328,6 +326,8 @@ static void parse_elf(void *output)
 	}
 
 	free(phdrs);
+
+	return ehdr.e_entry - LOAD_PHYSICAL_ADDR;
 }
 
 /*
@@ -356,6 +356,7 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
 	const unsigned long kernel_total_size = VO__end - VO__text;
 	unsigned long virt_addr = LOAD_PHYSICAL_ADDR;
 	unsigned long needed_size;
+	size_t entry_offset;
 
 	/* Retain x86 boot parameters pointer passed from startup_32/64. */
 	boot_params = rmode;
@@ -456,14 +457,17 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
 	debug_putstr("\nDecompressing Linux... ");
 	__decompress(input_data, input_len, NULL, NULL, output, output_len,
 			NULL, error);
-	parse_elf(output);
+	entry_offset = parse_elf(output);
 	handle_relocations(output, output_len, virt_addr);
-	debug_putstr("done.\nBooting the kernel.\n");
+
+	debug_putstr("done.\nBooting the kernel (entry_offset: 0x");
+	debug_puthex(entry_offset);
+	debug_putstr(").\n");
 
 	/* Disable exception handling before booting the kernel */
 	cleanup_exception_handling();
 
-	return output;
+	return output + entry_offset;
 }
 
 void fortify_panic(const char *name)