linux-kernel - Re: [PATCH] modpost: skip ELF local symbols during section mismatch check

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Date:   Thu, 22 Nov 2018 20:44:02 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     paul.walmsley@...ive.com
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>, jimw@...ive.com,
        Michal Marek <michal.lkml@...kovi.net>,
        Sam Ravnborg <sam@...nborg.org>, Paul Walmsley <paul@...an.com>
Subject: Re: [PATCH] modpost: skip ELF local symbols during section mismatch check

On Thu, Nov 22, 2018 at 4:11 PM Paul Walmsley <paul.walmsley@...ive.com> wrote:
>
>
> During development of a serial console driver with a gcc 8.2.0
> toolchain for RISC-V, the following modpost warning appeared:
>
> ----
> WARNING: vmlinux.o(.data+0x19b10): Section mismatch in reference from the variable .LANCHOR1 to the function .init.text:sifive_serial_console_setup()
> The variable .LANCHOR1 references
> the function __init sifive_serial_console_setup()
> If the reference is valid then annotate the
> variable with __init* or __refdata (see linux/init.h) or name the variable:
> *_template, *_timer, *_sht, *_ops, *_probe, *_probe_one, *_console
> ----
>
> ".LANCHOR1" is an ELF local symbol, automatically created by gcc's section
> anchor generation code:
>
> https://gcc.gnu.org/onlinedocs/gccint/Anchored-Addresses.html
>
> https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/varasm.c;h=cd9591a45617464946dcf9a126dde277d9de9804;hb=9fb89fa845c1b2e0a18d85ada0b077c84508ab78#l7473
>
> This was verified by compiling the kernel with -fno-section-anchors
> and observing that the ".LANCHOR1" ELF local symbol disappeared, and
> modpost no longer warned about the section mismatch.  The serial
> driver code idiom triggering the warning is standard Linux serial
> driver practice that has a specific whitelist inclusion in modpost.c.
>
> I'm neither a modpost nor an ELF expert, but naively, it doesn't seem
> useful for modpost to report section mismatch warnings caused by ELF
> local symbols by default.  Local symbols have compiler-generated
> names, and thus bypass modpost's whitelisting algorithm, which relies
> on the presence of a non-autogenerated symbol name.  This increases
> the likelihood that false positive warnings will be generated (as in
> the above case).
>
> Thus, disable section mismatch reporting on ELF local symbols.  The
> rationale here is similar to that of commit 2e3a10a1551d ("ARM: avoid
> ARM binutils leaking ELF local symbols") and of similar code already
> present in modpost.c:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/mod/modpost.c?h=v4.19-rc4&id=7876320f88802b22d4e2daf7eb027dd14175a0f8#n1256
>
> This third version of the patch implements a suggestion from Masahiro
> Yamada <yamada.masahiro@...ionext.com> to restructure the code as an
> additional pattern matching step inside secref_whitelist(), and
> further improves the patch description.
>
> Cc: Russell King <linux@...linux.org.uk>
> Cc: Jim Wilson <jimw@...ive.com>
> Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>
> Cc: Michal Marek <michal.lkml@...kovi.net>
> Cc: Sam Ravnborg <sam@...nborg.org>
> Cc: linux-kbuild@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Paul Walmsley <paul.walmsley@...ive.com>
> Signed-off-by: Paul Walmsley <paul@...an.com>
> ---

Applied with Sam's Ack.

Thanks!


>   scripts/mod/modpost.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
>
> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> index 0d998c54564d..90bb04b4e166 100644
> --- a/scripts/mod/modpost.c
> +++ b/scripts/mod/modpost.c
> @@ -1163,6 +1163,14 @@ static const struct sectioncheck *section_mismatch(
>    *   fromsec = text section
>    *   refsymname = *.constprop.*
>    *
> + * Pattern 6:
> + *   Hide section mismatch warnings for ELF local symbols.  The goal
> + *   is to eliminate false positive modpost warnings caused by
> + *   compiler-generated ELF local symbol names such as ".LANCHOR1".
> + *   Autogenerated symbol names bypass modpost's "Pattern 2"
> + *   whitelisting, which relies on pattern-matching against symbol
> + *   names to work.  (One situation where gcc can autogenerate ELF
> + *   local symbols is when "-fsection-anchors" is used.)
>    **/
>   static int secref_whitelist(const struct sectioncheck *mismatch,
>                             const char *fromsec, const char *fromsym,
> @@ -1201,6 +1209,10 @@ static int secref_whitelist(const struct sectioncheck *mismatch,
>             match(fromsym, optim_symbols))
>                 return 0;
>
> +       /* Check for pattern 6 */
> +       if (strstarts(fromsym, ".L"))
> +               return 0;
> +
>         return 1;
>   }
>
> --
> 2.19.1
>
>


-- 
Best Regards
Masahiro Yamada