Re: [PATCH v2] kbuild: keep symbols for symbol_get() even with CONFIG_TRIM_UNUSED_KSYMS

[Luis Chamberlain <mcgrof@kernel.org>]

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@linux-foundation.org>
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

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

  Luis