| Message ID | 87vc81lj7x.fsf@rustcorp.com.au (mailing list archive) |
|---|---|
| State | Superseded, archived |
| Headers | show |
On 04/05/13 06:00, Rusty Russell wrote: > Exactly. Don't workaround it here, revert it and put the > duplicate-section-name fixup in parisc where it belongs. > > Assuming parisc still produces these dup sections: that patch is 4 years > old now. > > Untested: > > diff --git a/arch/parisc/kernel/module.c b/arch/parisc/kernel/module.c > index 2a625fb..28d32a2 100644 > --- a/arch/parisc/kernel/module.c > +++ b/arch/parisc/kernel/module.c > @@ -341,6 +341,11 @@ int module_frob_arch_sections(CONST Elf_Ehdr *hdr, > ".PARISC.unwind", 14) == 0) > me->arch.unwind_section = i; > > + /* we produce multiple, empty .text sections, and kallsyms > + * gets upset. make non-alloc so it doesn't see them. */ > + if (sechdrs[i].sh_size == 0) > + sechdrs[i].sh_flags &= ~SHF_ALLOC; > + > if (sechdrs[i].sh_type != SHT_RELA) > continue; We just worked your suggested patch in. > Why? Does something refer to this empty section? Why has noone noticed > this since 2009? GDB wants to know all section with attribute ALLOC, regardless whether they are empty or not. Thus, it is useful if all of them appear in sysfs. > A zero-length section doesn't change the binary's structure. You don't > see non-SHF_ALLOC sections either. Yes, but they do occupy an index in the section headers of the binary. GDB needs to know all of them in the right order. -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Fri, 2013-04-05 at 14:30 +1030, Rusty Russell wrote: > Sebastian Wankerl <sisewank@cip.cs.fau.de> writes: > > On 04/04/13 03:00, Rusty Russell wrote: > >> Sebastian Wankerl <sisewank@cip.cs.fau.de> writes: > >>> Add non-zero module sections to sysfs on architectures unequal to PARISC. > >>> KGDB needs all module sections for proper module debugging. Therefore, commit > >>> 35dead4235e2b67da7275b4122fed37099c2f462 is revoked except for PARISC > >>> architecture. Thanks for actually cc'ing us. > >> #ifdef CONFIG_PARISC in the middle of kernel/module.c is super-ugly, and > >> wrong. > > > > I don't see why this is wrong. It used to load all sections to sysfs > > until the patch mentioned. Actually, it is the PARISC build chain which > > is broken. > > Exactly. Don't workaround it here, revert it and put the > duplicate-section-name fixup in parisc where it belongs. > > Assuming parisc still produces these dup sections: that patch is 4 years > old now. Just so you know: this isn't a parisc specific problem. Gcc produces duplicate section names under various circumstances, but the one that bites us is -ffunction-sections. Note that there are proposals to use -ffunction-sections on all architectures (so we can garbage collect unused functions) in which case you'll induce the bug identified in 35dead4235e2b67da7275b4122fed37099c2f462 on every architecture The problem is our assumption that section names be unique. This assumption is wrong. The ELF spec says (version 1.1 page 1-15): "An object file may have more than one section with the same name." We need to fix the kernel not to rely on a bogus assumption ... but we had no idea how to do that in a way that preserved the backwards compatibility of sections subdirectory. I admit that 35dead4235e2b67da7275b4122fed37099c2f462 is a hack, but now the problem has got attention, can we fix it properly? James -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
James Bottomley <James.Bottomley@HansenPartnership.com> writes: > On Fri, 2013-04-05 at 14:30 +1030, Rusty Russell wrote: >> Sebastian Wankerl <sisewank@cip.cs.fau.de> writes: >> > On 04/04/13 03:00, Rusty Russell wrote: >> >> Sebastian Wankerl <sisewank@cip.cs.fau.de> writes: >> >>> Add non-zero module sections to sysfs on architectures unequal to PARISC. >> >>> KGDB needs all module sections for proper module debugging. Therefore, commit >> >>> 35dead4235e2b67da7275b4122fed37099c2f462 is revoked except for PARISC >> >>> architecture. > > Thanks for actually cc'ing us. > >> >> #ifdef CONFIG_PARISC in the middle of kernel/module.c is super-ugly, and >> >> wrong. >> > >> > I don't see why this is wrong. It used to load all sections to sysfs >> > until the patch mentioned. Actually, it is the PARISC build chain which >> > is broken. >> >> Exactly. Don't workaround it here, revert it and put the >> duplicate-section-name fixup in parisc where it belongs. >> >> Assuming parisc still produces these dup sections: that patch is 4 years >> old now. > > Just so you know: this isn't a parisc specific problem. Gcc produces > duplicate section names under various circumstances, but the one that > bites us is -ffunction-sections. *This* is a PA-RISC specific issue. -ffunction-sections is a different problem, which this hack wouldn't help. > Note that there are proposals to use > -ffunction-sections on all architectures (so we can garbage collect > unused functions) in which case you'll induce the bug identified in > 35dead4235e2b67da7275b4122fed37099c2f462 on every architecture Good point, though I note that we seem to have stalled on -ffunction-sections. (And I vaguely recall an issue with -ffunction-sections and using ld -o which would fold duplicate named sections back together reducing elimination opportunities). > The problem is our assumption that section names be unique. This > assumption is wrong. The ELF spec says (version 1.1 page 1-15): "An > object file may have more than one section with the same name." We need > to fix the kernel not to rely on a bogus assumption ... but we had no > idea how to do that in a way that preserved the backwards compatibility > of sections subdirectory. > > I admit that 35dead4235e2b67da7275b4122fed37099c2f462 is a hack, but now > the problem has got attention, can we fix it properly? Yep. The original patch didn't go through me, or we would have had this discussion back then... The use of section names in sysfs goes back to one Mr. Corbet. Why did he do it that way? Because gdb's add-symbol-file makes the same assumption. So if we fixed the sysfs somehow, it still wouldn't be useful, since there's no way to tell gdb :( The real answer don't use -ffunction-sections on modules: probably not as important as the rest of the kernel. And the new shiny is -flto anyway. And that leaves us with a PA-RISC specific issue, for which we should move the fix to PA-RISC. Thoughts? Rusty. -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Hello. On Fri, Apr 05, 2013 at 12:07:15PM +0200, James Bottomley wrote: > Just so you know: this isn't a parisc specific problem. Gcc produces > duplicate section names under various circumstances, but the one that > bites us is -ffunction-sections. Note that there are proposals to use > -ffunction-sections on all architectures (so we can garbage collect > unused functions) in which case you'll induce the bug identified in > 35dead4235e2b67da7275b4122fed37099c2f462 on every architecture I am not able to produce an object file with duplicate section names using gcc on x86. Even with -ffunction-sections, every section gets a unique name. Is this architecture-specific behaviour of gcc? Greetings, Philip -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Sat, 2013-04-06 at 15:22 +1030, Rusty Russell wrote: > > The problem is our assumption that section names be unique. This > > assumption is wrong. The ELF spec says (version 1.1 page 1-15): "An > > object file may have more than one section with the same name." We need > > to fix the kernel not to rely on a bogus assumption ... but we had no > > idea how to do that in a way that preserved the backwards compatibility > > of sections subdirectory. > > > > I admit that 35dead4235e2b67da7275b4122fed37099c2f462 is a hack, but now > > the problem has got attention, can we fix it properly? > > Yep. The original patch didn't go through me, or we would have had this > discussion back then... > > The use of section names in sysfs goes back to one Mr. Corbet. Why did > he do it that way? Because gdb's add-symbol-file makes the same > assumption. So if we fixed the sysfs somehow, it still wouldn't be > useful, since there's no way to tell gdb :( > > The real answer don't use -ffunction-sections on modules: probably not > as important as the rest of the kernel. And the new shiny is > -flto anyway. > > And that leaves us with a PA-RISC specific issue, for which we should > move the fix to PA-RISC. > > Thoughts? Well, we don't have much of a choice. Our ELF stub jump on 32 bits is a PCREL17. That means once a module size is over 128k there's a chance we might not be able to link it because the jump is too big for the instruction. IPV6 is one such big module today, but I'm sure there are others. The only way I know to fix this is to allow the linker to insert stubs between functions, so we only fail at linking if a single function is >128k big. The way to do this is -ffunction-sections, unless there's something else we could do (all we really need is a way to ensure we can insert ELF stubs every 128k). We're not the only architecture that has these problems: frv, metag and score seem to as well. James -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 6-Apr-13, at 6:52 AM, James Bottomley wrote: > On Sat, 2013-04-06 at 15:22 +1030, Rusty Russell wrote: >>> The problem is our assumption that section names be unique. This >>> assumption is wrong. The ELF spec says (version 1.1 page 1-15): "An >>> object file may have more than one section with the same name." >>> We need >>> to fix the kernel not to rely on a bogus assumption ... but we had >>> no >>> idea how to do that in a way that preserved the backwards >>> compatibility >>> of sections subdirectory. >>> >>> I admit that 35dead4235e2b67da7275b4122fed37099c2f462 is a hack, >>> but now >>> the problem has got attention, can we fix it properly? >> >> Yep. The original patch didn't go through me, or we would have had >> this >> discussion back then... >> >> The use of section names in sysfs goes back to one Mr. Corbet. Why >> did >> he do it that way? Because gdb's add-symbol-file makes the same >> assumption. So if we fixed the sysfs somehow, it still wouldn't be >> useful, since there's no way to tell gdb :( >> >> The real answer don't use -ffunction-sections on modules: probably >> not >> as important as the rest of the kernel. And the new shiny is >> -flto anyway. >> >> And that leaves us with a PA-RISC specific issue, for which we should >> move the fix to PA-RISC. >> >> Thoughts? > > Well, we don't have much of a choice. Our ELF stub jump on 32 bits > is a > PCREL17. That means once a module size is over 128k there's a > chance we > might not be able to link it because the jump is too big for the > instruction. IPV6 is one such big module today, but I'm sure there > are > others. The only way I know to fix this is to allow the linker to > insert stubs between functions, so we only fail at linking if a single > function is >128k big. The way to do this is -ffunction-sections, > unless there's something else we could do (all we really need is a way > to ensure we can insert ELF stubs every 128k). There is now a config work around for this. See: http://www.spinics.net/lists/linux-parisc/msg04521.html Dave -- John David Anglin dave.anglin@bell.net -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
John David Anglin <dave.anglin@bell.net> wrote: >On 6-Apr-13, at 6:52 AM, James Bottomley wrote: > >> On Sat, 2013-04-06 at 15:22 +1030, Rusty Russell wrote: >>>> The problem is our assumption that section names be unique. This >>>> assumption is wrong. The ELF spec says (version 1.1 page 1-15): >"An >>>> object file may have more than one section with the same name." >>>> We need >>>> to fix the kernel not to rely on a bogus assumption ... but we had > >>>> no >>>> idea how to do that in a way that preserved the backwards >>>> compatibility >>>> of sections subdirectory. >>>> >>>> I admit that 35dead4235e2b67da7275b4122fed37099c2f462 is a hack, >>>> but now >>>> the problem has got attention, can we fix it properly? >>> >>> Yep. The original patch didn't go through me, or we would have had > >>> this >>> discussion back then... >>> >>> The use of section names in sysfs goes back to one Mr. Corbet. Why > >>> did >>> he do it that way? Because gdb's add-symbol-file makes the same >>> assumption. So if we fixed the sysfs somehow, it still wouldn't be >>> useful, since there's no way to tell gdb :( >>> >>> The real answer don't use -ffunction-sections on modules: probably >>> not >>> as important as the rest of the kernel. And the new shiny is >>> -flto anyway. >>> >>> And that leaves us with a PA-RISC specific issue, for which we >should >>> move the fix to PA-RISC. >>> >>> Thoughts? >> >> Well, we don't have much of a choice. Our ELF stub jump on 32 bits >> is a >> PCREL17. That means once a module size is over 128k there's a >> chance we >> might not be able to link it because the jump is too big for the >> instruction. IPV6 is one such big module today, but I'm sure there >> are >> others. The only way I know to fix this is to allow the linker to >> insert stubs between functions, so we only fail at linking if a >single >> function is >128k big. The way to do this is -ffunction-sections, >> unless there's something else we could do (all we really need is a >way >> to ensure we can insert ELF stubs every 128k). > >There is now a config work around for this. See: >http://www.spinics.net/lists/linux-parisc/msg04521.html The longcalls config option only works on pa2 doesn't it? Although we could just deprecate pa1. James
On 6-Apr-13, at 9:22 PM, James Bottomley wrote: > > > John David Anglin <dave.anglin@bell.net> wrote: > >> On 6-Apr-13, at 6:52 AM, James Bottomley wrote: >> >>> On Sat, 2013-04-06 at 15:22 +1030, Rusty Russell wrote: >>>>> The problem is our assumption that section names be unique. This >>>>> assumption is wrong. The ELF spec says (version 1.1 page 1-15): >> "An >>>>> object file may have more than one section with the same name." >>>>> We need >>>>> to fix the kernel not to rely on a bogus assumption ... but we had >> >>>>> no >>>>> idea how to do that in a way that preserved the backwards >>>>> compatibility >>>>> of sections subdirectory. >>>>> >>>>> I admit that 35dead4235e2b67da7275b4122fed37099c2f462 is a hack, >>>>> but now >>>>> the problem has got attention, can we fix it properly? >>>> >>>> Yep. The original patch didn't go through me, or we would have had >> >>>> this >>>> discussion back then... >>>> >>>> The use of section names in sysfs goes back to one Mr. Corbet. Why >> >>>> did >>>> he do it that way? Because gdb's add-symbol-file makes the same >>>> assumption. So if we fixed the sysfs somehow, it still wouldn't be >>>> useful, since there's no way to tell gdb :( >>>> >>>> The real answer don't use -ffunction-sections on modules: probably >>>> not >>>> as important as the rest of the kernel. And the new shiny is >>>> -flto anyway. >>>> >>>> And that leaves us with a PA-RISC specific issue, for which we >> should >>>> move the fix to PA-RISC. >>>> >>>> Thoughts? >>> >>> Well, we don't have much of a choice. Our ELF stub jump on 32 bits >>> is a >>> PCREL17. That means once a module size is over 128k there's a >>> chance we >>> might not be able to link it because the jump is too big for the >>> instruction. IPV6 is one such big module today, but I'm sure there >>> are >>> others. The only way I know to fix this is to allow the linker to >>> insert stubs between functions, so we only fail at linking if a >> single >>> function is >128k big. The way to do this is -ffunction-sections, >>> unless there's something else we could do (all we really need is a >> way >>> to ensure we can insert ELF stubs every 128k). >> >> There is now a config work around for this. See: >> http://www.spinics.net/lists/linux-parisc/msg04521.html > > The longcalls config option only works on pa2 doesn't it? Although > we could just deprecate pa1. No, it works on pa1.1 but the calls are more efficient on pa2. On linux with a flat space, they should be about the same in terms of instruction count. On HP-UX, an additional space register load is needed. For calls within the same space, the be instruction is pretty efficient but we never implemented linker support for it in binutils. If I recall correctly, it works if the call is local to a module but not for global calls. It does work in HP-UX. Dave -- John David Anglin dave.anglin@bell.net -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Philip Kranz <philip.kranz@googlemail.com> writes: > Hello. > > On Fri, Apr 05, 2013 at 12:07:15PM +0200, James Bottomley wrote: >> Just so you know: this isn't a parisc specific problem. Gcc produces >> duplicate section names under various circumstances, but the one that >> bites us is -ffunction-sections. Note that there are proposals to use >> -ffunction-sections on all architectures (so we can garbage collect >> unused functions) in which case you'll induce the bug identified in >> 35dead4235e2b67da7275b4122fed37099c2f462 on every architecture > > I am not able to produce an object file with duplicate section names > using gcc on x86. Even with -ffunction-sections, every section gets a > unique name. Is this architecture-specific behaviour of gcc? Good point. ld -r will collapse them into the same section (since gcc produces them they have to have the same section attributes). You can do it with --unique, but no arch uses that. PARISC has a platform-specific toolchain hack which does that for .text sections. (Thanks to Alan Modra for that clue...) Thanks, Rusty. -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Mon, Apr 08, 2013 at 01:44:45PM +0930, Rusty Russell wrote: > Philip Kranz <philip.kranz@googlemail.com> writes: > > I am not able to produce an object file with duplicate section names > > using gcc on x86. Even with -ffunction-sections, every section gets a > > unique name. Is this architecture-specific behaviour of gcc? > > Good point. ld -r will collapse them into the same section (since gcc > produces them they have to have the same section attributes). > > You can do it with --unique, but no arch uses that. PARISC has a > platform-specific toolchain hack which does that for .text sections. > (Thanks to Alan Modra for that clue...) So that problem is indeed platform-specific. If it is safe to assume that kernel modules don't have duplicate section names (except on PARISC), it would make sense to simply move the check for empty sections to arch/parisc as you suggested. James, what do you think about that? Greetings, Philip -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/arch/parisc/kernel/module.c b/arch/parisc/kernel/module.c index 2a625fb..28d32a2 100644 --- a/arch/parisc/kernel/module.c +++ b/arch/parisc/kernel/module.c @@ -341,6 +341,11 @@ int module_frob_arch_sections(CONST Elf_Ehdr *hdr, ".PARISC.unwind", 14) == 0) me->arch.unwind_section = i; + /* we produce multiple, empty .text sections, and kallsyms + * gets upset. make non-alloc so it doesn't see them. */ + if (sechdrs[i].sh_size == 0) + sechdrs[i].sh_flags &= ~SHF_ALLOC; + if (sechdrs[i].sh_type != SHT_RELA) continue;