module: treat exit sections the same as init sections when !CONFIG_MODULE_UNLOAD

Dynamic code patching (alternatives, jump_label and static_call) can
have sites in __exit code, even it __exit is never executed. Therefore
__exit must be present at runtime, at least for as long as __init code
is.

Additionally, for jump_label and static_call, the __exit sites must also
identify as within_module_init(), such that the infrastructure is aware
to never touch them after module init -- alternatives are only ran once
at init and hence don't have this particular constraint.

By making __exit identify as __init for MODULE_UNLOAD, the above is
satisfied.

So, when !CONFIG_MODULE_UNLOAD, the section ordering should look like the
following, with the .exit sections moved to the init region of the module.

Core section allocation order:
 	.text
 	.rodata
 	__ksymtab_gpl
 	__ksymtab_strings
 	.note.* sections
 	.bss
 	.data
 	.gnu.linkonce.this_module
 Init section allocation order:
 	.init.text
 	.exit.text
 	.symtab
 	.strtab

[jeyu: thanks to Peter Zijlstra for most of changelog]

Link: https://lore.kernel.org/lkml/YFiuphGw0RKehWsQ@gunter/
Link: https://lore.kernel.org/r/20210323142756.11443-1-jeyu@kernel.orgAcked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jessica Yu <jeyu@kernel.org>

kernel/module.c

@@ -2802,7 +2802,11 @@ void * __weak module_alloc(unsigned long size)
 
 bool __weak module_init_section(const char *name)
 {
+#ifndef CONFIG_MODULE_UNLOAD
+	return strstarts(name, ".init") || module_exit_section(name);
+#else
 	return strstarts(name, ".init");
+#endif
 }
 
 bool __weak module_exit_section(const char *name)
@@ -3116,11 +3120,6 @@ static int rewrite_section_headers(struct load_info *info, int flags)
 		 */
 		shdr->sh_addr = (size_t)info->hdr + shdr->sh_offset;
 
-#ifndef CONFIG_MODULE_UNLOAD
-		/* Don't load .exit sections */
-		if (module_exit_section(info->secstrings+shdr->sh_name))
-			shdr->sh_flags &= ~(unsigned long)SHF_ALLOC;
-#endif
 	}
 
 	/* Track but don't keep modinfo and version sections. */