Re: [PATCH v2 2/2] modpost: skip ELF local symbols by default during section mismatch check

[Masahiro Yamada]

Hi Paul,


On Thu, Nov 15, 2018 at 9:57 AM Paul Walmsley <paul.walmsley@xxxxxxxxxx> 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.
> The serial driver code idiom triggering the warning is standard serial
> driver practice, and one 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 second version of the patch drops the option to keep section
> mismatch warnings for local sections, based on feedback from Sam
> Ravnborg <sam@xxxxxxxxxxxx>; and clarifies that these warnings
> appear with gcc 8.2.0.
>
> Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> Cc: Jim Wilson <jimw@xxxxxxxxxx>
> Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> Cc: Michal Marek <michal.lkml@xxxxxxxxxxx>
> Cc: Sam Ravnborg <sam@xxxxxxxxxxxx>
> Cc: linux-kbuild@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Paul Walmsley <paul.walmsley@xxxxxxxxxx>
> Signed-off-by: Paul Walmsley <paul@xxxxxxxxx>
> ---
>  scripts/mod/modpost.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> index 85bd93c63180..0fb148171b78 100644
> --- a/scripts/mod/modpost.c
> +++ b/scripts/mod/modpost.c
> @@ -1253,6 +1253,17 @@ static inline int is_arm_mapping_symbol(const char *str)
>                && (str[2] == '\0' || str[2] == '.');
>  }
>
> +/*
> + * If a symbol name follows the convention for ELF-local symbols (i.e., the
> + * name begins with a ".L"), return true; otherwise false.  This is used to
> + * skip section mismatch reporting on ELF-local symbols, due to the risk of
> + * false positives.
> + */
> +static inline int is_local_symbol(const char *str)
> +{
> +       return str[0] == '.' && str[1] == 'L';
> +}
> +
>  /*
>   * If there's no name there, ignore it; likewise, ignore it if it's
>   * one of the magic symbols emitted used by current ARM tools.
> @@ -1535,6 +1546,9 @@ static void default_mismatch_handler(const char *modname, struct elf_info *elf,
>         if (strstarts(fromsym, "reference___initcall"))
>                 return;
>
> +       if (is_local_symbol(fromsym))
> +               return;
> +
>         tosec = sec_name(elf, get_secindex(elf, sym));
>         to = find_elf_symbol(elf, r->r_addend, sym);
>         tosym = sym_name(elf, to);
> --
> 2.19.1
>


I think this is almost good.
Just a nit.

Maybe, putting this check in secref_whitelist()
(with a comment "Pattern 6:")
could look more consistency.


Also, if you use strstart() helper,
you can remove is_local_symbol() function.


        /* Check for pattern 6 */
        if (strstarts(fromsym, ".L"))
                return 0;




-- 
Best Regards
Masahiro Yamada