linux-kernel - Re: [PATCH v2 2/6] livepatch: Add klp-convert tool

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Date: Wed, 22 May 2024 12:05:26 +0200
From: Petr Mladek <pmladek@...e.com>
To: Lukas Hruska <lhruska@...e.cz>
Cc: mbenes@...e.cz, jpoimboe@...nel.org, joe.lawrence@...hat.com,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-kbuild@...r.kernel.org, mpdesouza@...e.com,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v2 2/6] livepatch: Add klp-convert tool

On Thu 2024-05-16 15:30:05, Lukas Hruska wrote:
> Livepatches need to access external symbols which can't be handled
> by the normal relocation mechanism. It is needed for two types
> of symbols:
> 
>   + Symbols which can be local for the original livepatched function.
>     The alternative implementation in the livepatch sees them
>     as external symbols.
> 
>   + Symbols in modules which are exported via EXPORT_SYMBOL*(). They
>     must be handled special way otherwise the livepatch module would
>     depend on the livepatched one. Loading such livepatch would cause
>     loading the other module as well.
> 
> The address of these symbols can be found via kallsyms. Or they can
> be relocated using livepatch specific relocation sections as specified
> in Documentation/livepatch/module-elf-format.txt.
> 
> --- /dev/null
> +++ b/scripts/livepatch/klp-convert.c
> +/* Converts rela symbol names */
> +static bool convert_symbol(struct symbol *s)
> +{
> +	char lp_obj_name[MODULE_NAME_LEN];
> +	char sym_obj_name[MODULE_NAME_LEN];
> +	char sym_name[KSYM_NAME_LEN];
> +	char *klp_sym_name;
> +	unsigned long sym_pos;
> +	int poslen;
> +	unsigned int length;
> +
> +	static_assert(MODULE_NAME_LEN >= 56 && KSYM_NAME_LEN == 512,
> +			"Update limit in the below sscanf()");
> +
> +	if (sscanf(s->name, KLP_SYM_RELA_PREFIX "%55[^.].%55[^.].%511[^,],%lu",
> +			lp_obj_name, sym_obj_name, sym_name, &sym_pos) != 4) {
> +		WARN("Invalid format of symbol (%s)\n", s->name);
> +		return false;
> +	}
> +
> +	poslen = calc_digits(sym_pos);
> +
> +	length = strlen(KLP_SYM_PREFIX) + strlen(sym_obj_name)
> +		 + strlen(sym_name) + sizeof(poslen) + 3;

There should be "poslen" instead of "sizeof(poslen)".

> +
> +	klp_sym_name = calloc(1, length);
> +	if (!klp_sym_name) {
> +		WARN("Memory allocation failed (%s%s.%s,%lu)\n", KLP_SYM_PREFIX,
> +				sym_obj_name, sym_name, sym_pos);
> +		return false;
> +	}
> +
> +	if (safe_snprintf(klp_sym_name, length, KLP_SYM_PREFIX "%s.%s,%lu",
> +			  sym_obj_name, sym_name, sym_pos)) {
> +
> +		WARN("Length error (%s%s.%s,%lu)", KLP_SYM_PREFIX,
> +				sym_obj_name, sym_name, sym_pos);
> +		free(klp_sym_name);
> +		return false;
> +	}
> +
> +	s->name = klp_sym_name;
> +	s->sec = NULL;
> +	s->sym.st_name = -1;
> +	s->sym.st_shndx = SHN_LIVEPATCH;
> +
> +	return true;
> +}
> --- /dev/null
> +++ b/scripts/livepatch/klp-convert.h
> @@ -0,0 +1,23 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2016 Josh Poimboeuf <jpoimboe@...hat.com>
> + * Copyright (C) 2017 Joao Moreira   <jmoreira@...e.de>
> + *
> + */
> +
> +#define SHN_LIVEPATCH		0xff20
> +#define SHF_RELA_LIVEPATCH	0x00100000
> +#define MODULE_NAME_LEN		(64 - sizeof(GElf_Addr))
> +#define WARN(format, ...) \
> +	fprintf(stderr, "klp-convert: " format "\n", ##__VA_ARGS__)

Nit: I would remove "\n" here and add it to all callers. Half of the
     callers already have it ;-)

> +
> +/*
> + * klp-convert uses macros and structures defined in the linux sources
> + * package (see include/uapi/linux/livepatch.h). To prevent the
> + * dependency when building locally, they are defined below. Also notice
> + * that these should match the definitions from the targeted kernel.
> + */
> +
> +#define KLP_RELA_PREFIX			".klp.rela."
> +#define KLP_SYM_RELA_PREFIX		".klp.sym.rela."
> +#define KLP_SYM_PREFIX			".klp.sym."

Otherwise, it looks good.

Best Regards,
Petr