Hi Paul, On Sat, Oct 20, 2018 at 10:21 PM Paul Walmsley <paul.walmsley@xxxxxxxxxx> wrote: > > During development of a serial console driver with a RISC-V toolchain, > 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 > ---- Could you provide me a little more information to reproduce it? I tried your dev/paulw/serial-v4.19-rc7, but I could not get that warning. I used risc64-linux-gcc (GCC 7.3, 8.1) from kernel.org > ".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 by > default. The rationale here is similar to that of commit > 2e3a10a1551d6ceea005e6a62ca58183b8976217 ("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 > > Since it might be useful for some static analysis runs to detect > whether any new symbols have been added that result in any section > mismatches, even from ELF local symbols, this change can be disabled > (along with other tests that can often generate false positives) by > passing the '-P' switch on the modpost command line. > > Cc: Russell King <linux@xxxxxxxxxxxxxxx> > Cc: Jim Wilson <jimw@xxxxxxxxxx> > Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> > Cc: Michal Marek <michal.lkml@xxxxxxxxxxx> > 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 38fc1bd47926..2b8c960ece66 100644 > --- a/scripts/mod/modpost.c > +++ b/scripts/mod/modpost.c > @@ -1255,6 +1255,17 @@ 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. > @@ -1537,6 +1548,9 @@ static void default_mismatch_handler(const char *modname, struct elf_info *elf, > if (strstarts(fromsym, "reference___initcall")) > return; > > + if (!accept_falsepos_risk && 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 > -- Best Regards Masahiro Yamada
