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Message-ID: <lhupl8lxlpt.fsf@oldenburg.str.redhat.com>
Date: Thu, 11 Dec 2025 13:10:54 +0100
From: Florian Weimer <fweimer@...hat.com>
To: Bill Roberts <bill.roberts@....com>
Cc: musl@...ts.openwall.com
Subject: Re: [RFC 07/14] aarch64: rewrite tlsdesc reoutines in C
using inline asm
* Bill Roberts:
> Rewrite the AArch64 __tlsdesc_dynamic and __tlsdesc_static
> routines from assembly into implementations using inline assembly.
>
> This change eliminates the need for handwritten function prologues and
> epilogues in tlsdesc.s, which simplifies maintenance and allows the compiler
> to automatically insert architecture features such as BTI landing pads and
> pointer authentication (PAC) sequences where applicable.
>
> Moving to C also enables the compiler to manage register allocation,
> stack usage, and ABI compliance automatically while keeping the low-level
> behavior (bitmasks and register accesses) explicit and verifiable.
>
> No functional changes intended.
>
> Signed-off-by: Bill Roberts <bill.roberts@....com>
> ---
> src/ldso/aarch64/tlsdesc.c | 50 ++++++++++++++++++++++++++++++++++++++
> src/ldso/aarch64/tlsdesc.s | 31 -----------------------
> 2 files changed, 50 insertions(+), 31 deletions(-)
> create mode 100644 src/ldso/aarch64/tlsdesc.c
> delete mode 100644 src/ldso/aarch64/tlsdesc.s
>
> diff --git a/src/ldso/aarch64/tlsdesc.c b/src/ldso/aarch64/tlsdesc.c
> new file mode 100644
> index 00000000..224a9387
> --- /dev/null
> +++ b/src/ldso/aarch64/tlsdesc.c
> @@ -0,0 +1,50 @@
> +#include <stddef.h>
> +#include <stdint.h>
> +
> +/* size_t __tlsdesc_static(size_t *a) { return a[1]; } */
> +__attribute__((visibility("hidden")))
> +size_t __tlsdesc_static(size_t *a)
> +{
> + size_t result;
> +
> + __asm__ __volatile__(
> + "ldr %0, [%1, #8]\n\t" /* result = *(a + 8) */
> + : "=r"(result)
> + : "r"(a)
> + : "memory"
> + );
> +
> + return result;
> +}
I don't think these descriptor functions use the default AArch64 psABI,
so they can't be written in C (unless special compiler flags or function
attributes are used).
Thanks,
Florian
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