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Date: Tue, 23 Aug 2022 22:03:32 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: Ben Noordhuis <info@...ordhuis.nl>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] ldso: add basic ifunc support
* Ben Noordhuis <info@...ordhuis.nl> [2022-08-23 20:41:57 +0200]:
> Not perfect yet because it doesn't handle resolver functions that need
> relocations themselves but basic functionality works. That is, this
> works:
>
> typedef void (*Func)(void);
> void f(void) __attribute__((ifunc("g")));
> Func g(void) { return h; }
> void h(void) { abort(); }
>
> But returning an extern function does not:
>
> typedef void (*Func)(void);
> void f(void) __attribute__((ifunc("g")));
> Func g(void) { return abort; } // segfaults
there are a lot more problems than that.
the patch only works if f is hidden or binds locally in some other
way (e.g. in an executable) otherwise references to f will generate
symbolic relocations not IRELATIVE.
resolving symbolic relocation for ifunc symbols requires the ldso
to look at the symbol type (STT_GNU_IFUNC) and do the indirect call
based on that.
the main issue is that code is executed before the module is fully
relocated (in fact it may execute code in other libs before those are
relocated because symbol lookup can process not yet relcated libs)
which is problematic if the resolver function itself depends on
relocations (e.g. return h only works if constructing the address
of h does not require dynamic relocs or those relocs happen to get
resolved before the IRELATIVE, glibc tries a bit to shuffle ifunc
relocs to the end of reloc processing, it was never fixed completely
iirc because copy relocs and circular deps made things complicated).
and of course static linking is not addressed (where there are
further ordering issues since it is target dependent if ifunc has
to be resolved before or after TLS is set up).
there are further minor issues like the prototype of the resolver
is target dependent (usually hwcap is passed as argument) so the same
c code wont work across targets. or that ldd mode is not expected to
execute any code in the application but it is supposed to do reloc
processing (some might say this is a security issue).
> ---
> arch/x86_64/reloc.h | 1 +
> ldso/dynlink.c | 4 ++++
> src/internal/dynlink.h | 1 +
> 3 files changed, 6 insertions(+)
>
> diff --git a/arch/x86_64/reloc.h b/arch/x86_64/reloc.h
> index fac0c0ae..fb6be3ea 100644
> --- a/arch/x86_64/reloc.h
> +++ b/arch/x86_64/reloc.h
> @@ -10,6 +10,7 @@
> #define REL_DTPOFF R_X86_64_DTPOFF64
> #define REL_TPOFF R_X86_64_TPOFF64
> #define REL_TLSDESC R_X86_64_TLSDESC
> +#define REL_IRELATIVE R_X86_64_IRELATIVE
>
> #define CRTJMP(pc,sp) __asm__ __volatile__( \
> "mov %1,%%rsp ; jmp *%0" : : "r"(pc), "r"(sp) : "memory" )
> diff --git a/ldso/dynlink.c b/ldso/dynlink.c
> index fd09ca69..659cb113 100644
> --- a/ldso/dynlink.c
> +++ b/ldso/dynlink.c
> @@ -507,6 +507,10 @@ static void do_relocs(struct dso *dso, size_t *rel, size_t rel_size, size_t stri
> reloc_addr[1] = tmp;
> #endif
> break;
> + case REL_IRELATIVE:
> + *reloc_addr = (size_t)base + addend;
> + *reloc_addr = ((size_t(*)(void))*reloc_addr)();
> + break;
> default:
> error("Error relocating %s: unsupported relocation type %d",
> dso->name, type);
> diff --git a/src/internal/dynlink.h b/src/internal/dynlink.h
> index 830354eb..c67bc35e 100644
> --- a/src/internal/dynlink.h
> +++ b/src/internal/dynlink.h
> @@ -41,6 +41,7 @@ enum {
> REL_TPOFF,
> REL_TPOFF_NEG,
> REL_TLSDESC,
> + REL_IRELATIVE,
> REL_FUNCDESC,
> REL_FUNCDESC_VAL,
> };
> --
> 2.34.1
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