On Thu, Nov 22, 2018 at 4:11 PM 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 > 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@xxxxxxxxxxxxx> to restructure the code as an > additional pattern matching step inside secref_whitelist(), and > further improves the patch description. > > 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> > --- 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
