linux-kernel - Re: [PATCH v3] aarch64: vdso: Wire up getrandom() vDSO implementation

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Message-ID: <ZtXOmHYuTI9DQGij@zx2c4.com>
Date: Mon, 2 Sep 2024 16:41:28 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Mark Rutland <mark.rutland@....com>
Cc: Adhemerval Zanella <adhemerval.zanella@...aro.org>,
	Theodore Ts'o <tytso@....edu>, linux-kernel@...r.kernel.org,
	linux-crypto@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-arch@...r.kernel.org,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Eric Biggers <ebiggers@...nel.org>,
	Christophe Leroy <christophe.leroy@...roup.eu>
Subject: Re: [PATCH v3] aarch64: vdso: Wire up getrandom() vDSO implementation

On Mon, Sep 02, 2024 at 03:31:43PM +0100, Mark Rutland wrote:
> ssize_t __kernel_getrandom(void *buffer, size_t len, unsigned int flags,
> 			   void *opaque_state, size_t opaque_len)
> {
> 	if (alternative_has_cap_likely(ARM64_HAS_FPSIMD)) {
> 		return __cvdso_getrandom(buffer, len, flags,
> 					 opaque_state, opaque_len);
> 	}
> 
> 	if (unlikely(opaque_len == ~0UL && !buffer && !len && !flags))
> 		return -ENOSYS;
> 	
> 	return getrandom_syscall(buffer, len, flags);
> }
> 
> ... though the conditions for returning -ENOSYS look very odd to me; why
> do we care about fast-pathing that specific case rather than forwarding
> that to the kernel, and does __cvdso_getrandom() handle that correctly?

Adhemerval's code here is fine and correct. The opaque_len==~0UL thing
is a special vDSO case for getting the param struct back, not something
related to the kernel. See __cvdso_getrandom_data() for details.