Message ID | CAKv+Gu-wqoQVXvZcEF8xfAnHmrakKU73ToMNO+2Lw158B4teJA@mail.gmail.com (mailing list archive) |
---|---|
State | Not Applicable |
Headers | show |
On Wed, Jan 18, 2017 at 2:37 PM, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote: > > For a ballpark number of 10,000 CRCs in the core kernel, this would > increase the size of the image by 40 KB for 32-bit architectures (and > if saving 40 KB is essential, chances are you won't be using > modversions in the first place). As you say, I don't think the space issue is much of a problem. I'm more worried about the replacement of one crazy model that has problems due to linker subtlety with _another_ one. Your genksyms.c change is not exactly obvious. I looked at it, and my brain just shut down. Why both the LONG(0x%08lx); _and_ the "%s__crc_%s = 0x%08lx;\n" in the linker script? I'm sure there's a good reason, but I'd like to see a more explicit explanation fo what the generated linker script does and what the rules are. Linus
On 19 January 2017 at 00:15, Linus Torvalds <torvalds@linux-foundation.org> wrote: > On Wed, Jan 18, 2017 at 2:37 PM, Ard Biesheuvel > <ard.biesheuvel@linaro.org> wrote: >> >> For a ballpark number of 10,000 CRCs in the core kernel, this would >> increase the size of the image by 40 KB for 32-bit architectures (and >> if saving 40 KB is essential, chances are you won't be using >> modversions in the first place). > > As you say, I don't think the space issue is much of a problem. > > I'm more worried about the replacement of one crazy model that has > problems due to linker subtlety with _another_ one. > > Your genksyms.c change is not exactly obvious. I looked at it, and my > brain just shut down. Why both the > > LONG(0x%08lx); > > _and_ the > > "%s__crc_%s = 0x%08lx;\n" > > in the linker script? I'm sure there's a good reason, but I'd like to > see a more explicit explanation fo what the generated linker script > does and what the rules are. > This is simply because modpost still uses the value of the symbol rather than the value it points to to generate the /other/ side of the comparison (i.e., Module.symvers etc) I will look into updating modpost to dereference the symbol as well, and update the RFC patch accordingly.
On Thu, Jan 19, 2017 at 1:22 AM, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote: >> >> Your genksyms.c change is not exactly obvious. I looked at it, and my >> brain just shut down. Why both the >> >> LONG(0x%08lx); >> >> _and_ the >> >> "%s__crc_%s = 0x%08lx;\n" >> >> in the linker script? I'm sure there's a good reason, but I'd like to >> see a more explicit explanation fo what the generated linker script >> does and what the rules are. > > This is simply because modpost still uses the value of the symbol > rather than the value it points to to generate the /other/ side of the > comparison (i.e., Module.symvers etc) Ahh, now that you explained it, it was obvious. Thanks. But yes, I don't think we want that "both belt and suspenders" approach, so your updated patch that does things just one way is I think the right way. > I will look into updating modpost to dereference the symbol as well, > and update the RFC patch accordingly. Yes, so your updated patch looks good to me. I think our old "symbol with an absolute value" model was simpler conceptually, but given the existing absolute (sic) braindamage of linkers, I think your latest patch is probably the way to go. If for no other reason than the fact that it doesn't depend on something that clearly nobody else uses, and even the linker people were confused about. So I think the slightly more complex model of relative offsets is the simpler one in the end if it means that we don't have to have completely insane workarounds for linker damage. But maybe somebody else wants to pipe up. Preferably somebody who doesn't hate the symversions code as much as I do by now, and actually _uses_ it ;) Linus
On 19 January 2017 at 17:24, Linus Torvalds <torvalds@linux-foundation.org> wrote: > On Thu, Jan 19, 2017 at 1:22 AM, Ard Biesheuvel > <ard.biesheuvel@linaro.org> wrote: >>> >>> Your genksyms.c change is not exactly obvious. I looked at it, and my >>> brain just shut down. Why both the >>> >>> LONG(0x%08lx); >>> >>> _and_ the >>> >>> "%s__crc_%s = 0x%08lx;\n" >>> >>> in the linker script? I'm sure there's a good reason, but I'd like to >>> see a more explicit explanation fo what the generated linker script >>> does and what the rules are. >> >> This is simply because modpost still uses the value of the symbol >> rather than the value it points to to generate the /other/ side of the >> comparison (i.e., Module.symvers etc) > > Ahh, now that you explained it, it was obvious. Thanks. > > But yes, I don't think we want that "both belt and suspenders" > approach, so your updated patch that does things just one way is I > think the right way. > >> I will look into updating modpost to dereference the symbol as well, >> and update the RFC patch accordingly. > > Yes, so your updated patch looks good to me. > > I think our old "symbol with an absolute value" model was simpler > conceptually, but given the existing absolute (sic) braindamage of > linkers, I think your latest patch is probably the way to go. > OK. > If for no other reason than the fact that it doesn't depend on > something that clearly nobody else uses, and even the linker people > were confused about. > > So I think the slightly more complex model of relative offsets is the > simpler one in the end if it means that we don't have to have > completely insane workarounds for linker damage. > > But maybe somebody else wants to pipe up. Preferably somebody who > doesn't hate the symversions code as much as I do by now, and actually > _uses_ it ;) > I am not crazy about it either: I am simply trying to get rid of the ~10,000 pointless relocations in the arm64 KASLR kernel rather than having to rely on the dodgy code that 'repairs' the CRCs at runtime. I have noticed one slight snag though: the ARM module loader currently has no support for the R_ARM_REL32 relocations that are emitted by the relative references in the kcrctabs. I looked at other arches, x86, ia64, s390, power and arm64, and those all seem to be fully equipped in this regard, but it would be good if we could get some coverage for this code on other architectures to find out which ones need to have this support added as well. Thanks, Ard.
diff --git a/scripts/genksyms/genksyms.c b/scripts/genksyms/genksyms.c index 06121ce524a7..9f739c7224c3 100644 --- a/scripts/genksyms/genksyms.c +++ b/scripts/genksyms/genksyms.c @@ -693,7 +693,10 @@ void export_symbol(const char *name) fputs(">\n", debugfile); /* Used as a linker script. */ - printf("%s__crc_%s = 0x%08lx ;\n", mod_prefix, name, crc); + printf("SECTIONS { .rodata.__crc_%s : ALIGN(4) " + "{ %s__crcp_%s = .; LONG(0x%08lx); } }\n" + "%s__crc_%s = 0x%08lx;\n", + name, mod_prefix, name, crc, mod_prefix, name, crc); } } diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h index e3508a3d6e53..5e95a552a871 100644 --- a/include/asm-generic/export.h +++ b/include/asm-generic/export.h @@ -49,8 +49,8 @@ KSYM(__kstrtab_\name): .section ___kcrctab\sec+\name,"a" .balign KCRC_ALIGN KSYM(__kcrctab_\name): - .long KSYM(__crc_\name) - .weak KSYM(__crc_\name) + .long KSYM(__crcp_\name) - . + .weak KSYM(__crcp_\name) .previous #endif #endif