linux-kernel - Re: [PATCH v8 04/13] module: Move livepatch support to a separate file

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Date:   Fri, 25 Feb 2022 10:34:11 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Aaron Tomlin <atomlin@...hat.com>
Cc:     mcgrof@...nel.org, christophe.leroy@...roup.eu, cl@...ux.com,
        mbenes@...e.cz, akpm@...ux-foundation.org, jeyu@...nel.org,
        linux-kernel@...r.kernel.org, linux-modules@...r.kernel.org,
        void@...ifault.com, atomlin@...mlin.com, allen.lkml@...il.com,
        joe@...ches.com, msuchanek@...e.de, oleksandr@...alenko.name
Subject: Re: [PATCH v8 04/13] module: Move livepatch support to a separate
 file

On Tue 2022-02-22 14:12:54, Aaron Tomlin wrote:
> No functional change.
> 
> This patch migrates livepatch support (i.e. used during module
> add/or load and remove/or deletion) from core module code into
> kernel/module/livepatch.c. At the moment it contains code to
> persist Elf information about a given livepatch module, only.
> 
> Signed-off-by: Aaron Tomlin <atomlin@...hat.com>

> diff --git a/kernel/module/livepatch.c b/kernel/module/livepatch.c
> new file mode 100644
> index 000000000000..486d4ff92719
> --- /dev/null
> +++ b/kernel/module/livepatch.c
> @@ -0,0 +1,74 @@
> + * Persist Elf information about a module. Copy the Elf header,
> + * section header table, section string table, and symtab section
> + * index from info to mod->klp_info.
> + */
> +int copy_module_elf(struct module *mod, struct load_info *info)
> +{
> +	unsigned int size, symndx;
> +	int ret;
> +
> +	size = sizeof(*mod->klp_info);
> +	mod->klp_info = kmalloc(size, GFP_KERNEL);
> +	if (!mod->klp_info)
> +		return -ENOMEM;
> +
> +	/* Elf header */
> +	size = sizeof(mod->klp_info->hdr);
> +	memcpy(&mod->klp_info->hdr, info->hdr, size);
> +
> +	/* Elf section header table */
> +	size = sizeof(*info->sechdrs) * info->hdr->e_shnum;
> +	mod->klp_info->sechdrs = kmemdup(info->sechdrs, size, GFP_KERNEL);
> +	if (!mod->klp_info->sechdrs) {
> +		ret = -ENOMEM;
> +		goto free_info;
> +	}
> +
> +	/* Elf section name string table */
> +	size = info->sechdrs[info->hdr->e_shstrndx].sh_size;
> +	mod->klp_info->secstrings = kmemdup(info->secstrings, size, GFP_KERNEL);
> +	if (!mod->klp_info->secstrings) {
> +		ret = -ENOMEM;
> +		goto free_sechdrs;
> +	}
> +
> +	/* Elf symbol section index */
> +	symndx = info->index.sym;
> +	mod->klp_info->symndx = symndx;
> +
> +	/*
> +	 * For livepatch modules, core_kallsyms.symtab is a complete
> +	 * copy of the original symbol table. Adjust sh_addr to point
> +	 * to core_kallsyms.symtab since the copy of the symtab in module
> +	 * init memory is freed at the end of do_init_module().
> +	 */
> +	mod->klp_info->sechdrs[symndx].sh_addr = (unsigned long)mod->core_kallsyms.symtab;
> +
> +	return 0;

This code include several other well hidden changes:

--- del.p	2022-02-24 16:55:26.570054922 +0100
+++ add.p	2022-02-24 16:56:04.766781394 +0100
@@ -3,14 +3,14 @@
  * section header table, section string table, and symtab section
  * index from info to mod->klp_info.
  */
-static int copy_module_elf(struct module *mod, struct load_info *info)
+int copy_module_elf(struct module *mod, struct load_info *info)
 {
 	unsigned int size, symndx;
 	int ret;
 
 	size = sizeof(*mod->klp_info);
 	mod->klp_info = kmalloc(size, GFP_KERNEL);
-	if (mod->klp_info == NULL)
+	if (!mod->klp_info)
 		return -ENOMEM;
 
 	/* Elf header */
@@ -20,7 +20,7 @@ static int copy_module_elf(struct module
 	/* Elf section header table */
 	size = sizeof(*info->sechdrs) * info->hdr->e_shnum;
 	mod->klp_info->sechdrs = kmemdup(info->sechdrs, size, GFP_KERNEL);
-	if (mod->klp_info->sechdrs == NULL) {
+	if (!mod->klp_info->sechdrs) {
 		ret = -ENOMEM;
 		goto free_info;
 	}
@@ -28,7 +28,7 @@ static int copy_module_elf(struct module
 	/* Elf section name string table */
 	size = info->sechdrs[info->hdr->e_shstrndx].sh_size;
 	mod->klp_info->secstrings = kmemdup(info->secstrings, size, GFP_KERNEL);
-	if (mod->klp_info->secstrings == NULL) {
+	if (!mod->klp_info->secstrings) {
 		ret = -ENOMEM;
 		goto free_sechdrs;
 	}
@@ -43,8 +43,7 @@ static int copy_module_elf(struct module
 	 * to core_kallsyms.symtab since the copy of the symtab in module
 	 * init memory is freed at the end of do_init_module().
 	 */
-	mod->klp_info->sechdrs[symndx].sh_addr = \
-		(unsigned long) mod->core_kallsyms.symtab;
+	mod->klp_info->sechdrs[symndx].sh_addr = (unsigned long)mod->core_kallsyms.symtab;
 
 	return 0;


Please do not do these small coding style changes. It complicates the
review and increases the risk of regressions. Different people
have different preferences. Just imagine that every half a year
someone update style of a code by his personal preferences. The
real changes will then get lost in a lot of noise.

Coding style changes might be acceptable only when the code is
reworked or when it significantly improves readability.


That said. I reviewed and tested this patch and did not find any
problem. Feel free to use:

Reviewed-by: Petr Mladek <pmladek@...e.com>
Tested-by: Petr Mladek <pmladek@...e.com>

Please, take the above as an advice for your future work.

Best Regards,
Petr