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Donenfeld" To: Christophe Leroy Cc: Michael Ellerman , Nicholas Piggin , Naveen N Rao , Andy Lutomirski , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Theodore Ts'o , Arnd Bergmann , Andrew Morton , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Vincenzo Frascino , Shuah Khan , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v2 06/17] vdso: Change getrandom's generation to unsigned long Message-ID: References: <525b48eb79978ddba2d1b8ee23b27bd6c5b0b4ee.1724309198.git.christophe.leroy@csgroup.eu> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <525b48eb79978ddba2d1b8ee23b27bd6c5b0b4ee.1724309198.git.christophe.leroy@csgroup.eu> X-Rspamd-Server: rspam03 X-Rspam-User: X-Rspamd-Queue-Id: C6353180007 X-Stat-Signature: t5kuox57b4q4ghymm3pss3a3oirnqn9u X-HE-Tag: 1724658633-822807 X-HE-Meta: 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 3xXF6MtY 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Aug 22, 2024 at 09:13:14AM +0200, Christophe Leroy wrote: > Performing SMP atomic operations on u64 fails on powerpc32. > > Random driver generation is handled as unsigned long not u64, > see for instance base_cnrg or struct crng. > > Use the same type for vDSO's getrandom as it gets copied > from the above. This is also in line with the local > current_generation which is already an unsigned long. This isn't going to work when 32-bit userspace tries to access a 64-bit kernel. I had "fixed" this with a vdso_kernel_ulong type way back in an earlier version: https://lore.kernel.org/lkml/20240528122352.2485958-5-Jason@zx2c4.com/#Z31include:vdso:types.h But tglx pointed out in that thread that this actually isn't necessary: | All of this is pointless because if a 32-bit application runs on a | 64-bit kernel it has to use the 64-bit 'generation'. So why on earth do | we need magic here for a 32-bit kernel? | | Just use u64 for both and spare all this voodoo. We're seriously not | "optimizing" for 32-bit kernels. | | All what happens on a 32-bit kernel is that the RNG will store the | unsigned long (32bit) generation into a 64bit variable: | | smp_store_release(&_vdso_rng_data.generation, next_gen + 1); | | As the upper 32bit are always zero, there is no issue vs. load store | tearing at all. So there is zero benefit for this aside of slightly | "better" user space code when running on a 32-bit kernel. Who cares? So I just got rid of it and used a u64 as he suggested. However, there's also an additional reason why it's not worth churning further over this - because VM_DROPPABLE is 64-bit only (due to flags in vma bits), likely so is vDSO getrandom() for the time being. So I think it makes more sense to retool this series to be ppc64, and then if you really really want 32-bit and can convince folks it matters, then all of these parts (for example, here, the fact that the smp helper doesn't want to tear) can be fixed up in a separate series. Jason