linux-kernel - Re: [PATCH v2] kbuild: keep symbols for symbol_get() even with CONFIG_TRIM_UNUSED

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Message-ID: <Z2BcFZ89wCLHm8YW@bombadil.infradead.org>
Date: Mon, 16 Dec 2024 08:57:57 -0800
From: Luis Chamberlain <mcgrof@...nel.org>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>, Daniel Gomez <da.gomez@...sung.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>,
	Petr Pavlu <petr.pavlu@...e.com>,
	Sami Tolvanen <samitolvanen@...gle.com>, linux-arch@...r.kernel.org,
	linux-modules@...r.kernel.org
Subject: Re: [PATCH v2] kbuild: keep symbols for symbol_get() even with
 CONFIG_TRIM_UNUSED_KSYMS

On Sat, Dec 14, 2024 at 07:57:20PM +0900, Masahiro Yamada wrote:
> Linus observed that the symbol_request(utf8_data_table) call fails when
> CONFIG_UNICODE=y and CONFIG_TRIM_UNUSED_KSYMS=y.
> 
> symbol_get() relies on the symbol data being present in the ksymtab for
> symbol lookups. However, EXPORT_SYMBOL_GPL(utf8_data_table) is dropped
> due to CONFIG_TRIM_UNUSED_KSYMS, as no module references it in this case.
> 
> Probably, this has been broken since commit dbacb0ef670d ("kconfig option
> for TRIM_UNUSED_KSYMS").
> 
> This commit addresses the issue by leveraging modpost. Symbol names
> passed to symbol_get() are recorded in the special .no_trim_symbol
> section, which is then parsed by modpost to forcibly keep such symbols.
> The .no_trim_symbol section is discarded by the linker scripts, so there
> is no impact on the size of the final vmlinux or modules.
> 
> This commit cannot resolve the issue for direct calls to __symbol_get()
> because the symbol name is not known at compile-time.
> 
> Although symbol_get() may eventually be deprecated, this workaround
> should be good enough meanwhile.
> 
> Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>

Reviewed-by: Luis Chamberlain <mcgrof@...nel.org>

  Luis