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Date: Fri, 6 Oct 2023 00:20:25 -0400
From: Rich Felker <dalias@...c.org>
To: Rui Ueyama <rui314@...il.com>
Cc: musl@...ts.openwall.com
Subject: Re: arm32 tlsdesc bug
On Fri, Oct 06, 2023 at 01:08:18PM +0900, Rui Ueyama wrote:
> Hi,
>
> I think there's a bug in musl's TLSDESC implementation for ARM32.
>
> TLSDESC uses two consecutive GOT slots to store a function pointer and its
> argument. Usually, the function pointer is stored in the first slot and the
> argument in the second. However, on ARM32, the order is reversed; the
> argument is stored in the first slot.
>
> If a TLSDESC relocation has a non-zero addend, it's applied to the function
> argument and not to the function pointer. That means, for an ABI that uses
> the REL-type relocations (as opposed to RELA-type), the addend should be
> stored to the location where the function argument is stored, and that's
> the first slot on ARM32.
>
> So, I believe we need something like this.
>
> diff --git a/ldso/dynlink.c b/ldso/dynlink.c
> index ceca3c98..254fa5b8 100644
> --- a/ldso/dynlink.c
> +++ b/ldso/dynlink.c
> @@ -513,11 +513,17 @@ static void do_relocs(struct dso *dso, size_t *rel,
> size_t rel_size, size_t stri
> case REL_TPOFF_NEG:
> *reloc_addr = def.dso->tls.offset - tls_val +
> addend;
> break;
> #endif
> case REL_TLSDESC:
> - if (stride<3) addend = reloc_addr[1];
> + if (stride<3) {
> +#ifdef TLSDESC_BACKWARDS
> + addend = reloc_addr[0];
> +#else
> + addend = reloc_addr[1];
> +#endif
> + }
> if (def.dso->tls_id > static_tls_cnt) {
> struct td_index *new = malloc(sizeof *new);
> if (!new) {
> error(
> "Error relocating %s: cannot
> allocate TLSDESC for %s",
Thank you!! This almost surely explains the TLSDESC problems we've
encountered on arm (32-bit) that prevented enabling it by default.
Rich
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