From: Rich Felker <dalias@libc.org>
To: musl@lists.openwall.com
Subject: Re: ppc soft-float regression
Date: Fri, 22 May 2015 02:23:46 -0400 [thread overview]
Message-ID: <20150522062346.GK17573@brightrain.aerifal.cx> (raw)
In-Reply-To: <20150518220731.GA31132@euler>
[-- Attachment #1: Type: text/plain, Size: 3034 bytes --]
On Tue, May 19, 2015 at 12:07:31AM +0200, Felix Janda wrote:
> Rich Felker wrote:
> > On Mon, May 18, 2015 at 04:10:43PM -0400, Rich Felker wrote:
> > > OK I've looked at this and I understand what's happening. PowerPC does
> > > not have a separate relocation type for GOT entries; instead it uses
> > > the same relocation type used for address constants global data. These
> > > do not get re-processed after the main program and libraries are
> > > added, because unlike GOT slots, they have addends, and if the addend
> > > is inline (using REL rather than RELA) then it's already been
> > > clobbered by the early relocation phase and can't easily be recovered.
> > >
> > > I see three possible solutions:
> > >
> > > 1. Treat R_PPC_ADDR32 as a GOT relocation instead of a regular
> > > symbolic relocation in data. This would suppress the addend (giving
> > > wrong address) if inline addends (REL) were used, but in practice
> > > powerpc aways uses RELA. I consider this a hack, and perhaps risky,
> > > since in principle someone could make powerpc binaries with REL.
> > >
> > > 2. Re-process not just GOT type relocs, but also any RELA
> > > (non-inline-addend) relocs again on the second pass. This would
> > > work as long as powerpc only uses RELA, and if REL is ever used,
> > > the worst that would happen is the current bug (losing environ,
> > > etc.) rather than silently wrong relocations in global data. This
> > > approach is not a hack, but I consider it something of an
> > > incomplete fix.
> > >
> > > 3. Re-process all symbolic relocations. For REL-type (inline addend),
> > > we have to recover the original addend, which can be done by
> > > calling find_sym again, but using ldso instead of the current
> > > library chain head as the context to search for the symbol in, then
> > > subtracting the resulting address to get back the original addend.
> > >
> > > I like the third solution best, even though it incurs a small code
> > > size cost and a performance cost for archs using REL, because it's
> > > completely robust against any weird ways some archs might end up using
> > > relocations. The expected number of such relocations is tiny anyway;
> > > on my i386 builds it's 14.
> > >
> > > If option 3 proves to be difficult or costly, however, we could
> > > consider option 2 as a temporary measure to get powerpc working. It
> > > wouldn't even need to be reverted, because option 3 includes/subsumes
> > > the work that would be done for option 2.
> >
> > Attached is a patch to implement option 2. I'll probably commit it
> > soon anyway but here is it in case you want to test sooner. I verified
> > it fixes the test program on powerpc for me.
>
> Thanks for the quick fix! The new commit fixes also the other segfaults
> I've seen.
Attached is a patch that finishes the job by completing option 3. I
haven't tested it much yet so I'll hold off on committing it for a
while but it seems to work fine (not break anything) on i386.
Rich
[-- Attachment #2: reprocess_all_syms.diff --]
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diff --git a/src/ldso/dynlink.c b/src/ldso/dynlink.c
index 93595a0..485bd4f 100644
--- a/src/ldso/dynlink.c
+++ b/src/ldso/dynlink.c
@@ -280,12 +280,17 @@ static void do_relocs(struct dso *dso, size_t *rel, size_t rel_size, size_t stri
def.dso = dso;
}
- int gotplt = (type == REL_GOT || type == REL_PLT);
- if (dso->rel_update_got && !gotplt && stride==2) continue;
-
- addend = stride>2 ? rel[2]
- : gotplt || type==REL_COPY ? 0
- : *reloc_addr;
+ if (stride > 2) {
+ addend = rel[2];
+ } else if (type==REL_GOT || type==REL_PLT || type==REL_COPY) {
+ addend = 0;
+ } else {
+ addend = *reloc_addr;
+ if (dso->rel_update_got) {
+ struct symdef old = find_sym(&ldso, name, 0);
+ addend -= (size_t)ldso.base+old.sym->st_value;
+ }
+ }
sym_val = def.sym ? (size_t)def.dso->base+def.sym->st_value : 0;
tls_val = def.sym ? def.sym->st_value : 0;
next prev parent reply other threads:[~2015-05-22 6:23 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-17 8:03 Waldemar Brodkorb
2015-05-17 10:02 ` Felix Janda
2015-05-17 16:37 ` Rich Felker
2015-05-17 17:50 ` Felix Janda
2015-05-17 18:15 ` Felix Janda
2015-05-17 19:56 ` Felix Janda
2015-05-18 18:39 ` Felix Janda
2015-05-18 20:10 ` Rich Felker
2015-05-18 20:14 ` Rich Felker
2015-05-18 22:07 ` Felix Janda
2015-05-22 6:23 ` Rich Felker [this message]
2015-05-24 3:08 ` Rich Felker
2015-05-25 0:36 ` Rich Felker
2015-05-25 6:31 ` Jens Gustedt
2015-05-25 6:57 ` Rich Felker
2015-05-25 7:44 ` Jens Gustedt
2015-05-25 13:26 ` Szabolcs Nagy
2015-05-25 13:40 ` Alexander Monakov
2015-05-25 14:35 ` Szabolcs Nagy
2015-05-25 14:45 ` Alexander Monakov
2015-05-25 21:45 ` Rich Felker
2015-05-25 22:46 ` Rich Felker
2015-05-25 23:51 ` Rich Felker
2015-05-17 13:06 ` Felix Janda
2015-05-17 16:35 ` Rich Felker
2015-05-17 17:20 ` Szabolcs Nagy
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