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> --- 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
