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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43fe4e3a341sm64204586f8f.24.2026.04.24.02.26.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Apr 2026 02:26:46 -0700 (PDT) Date: Fri, 24 Apr 2026 10:26:45 +0100 From: David Laight To: Zong Li Cc: pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, debug@rivosinc.com, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] riscv: cif: reduce shadow stack size limit from 4GB to 2GB Message-ID: <20260424102645.14fa41cd@pumpkin> In-Reply-To: <20260424065540.3480755-1-zong.li@sifive.com> References: <20260424065540.3480755-1-zong.li@sifive.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 23 Apr 2026 23:55:40 -0700 Zong Li wrote: > Follow the ARM64 GCS (Guarded Control Stack) implementation approach > by reducing the shadow stack size allocation from min(RLIMIT_STACK, 4GB) > to min(RLIMIT_STACK/2, 2GB). see commit '506496bcbb42 "arm64/gcs: Ensure > that new threads have a GCS")' > > Rationale: > > 1. Shadow stacks only store return addresses (8 bytes per entry), not > local variables, function parameters, or saved registers. A 2GB > shadow stack is far more than sufficient for any practical > application, even with extremely deep recursion. Using half the size > maintains adequate while being more resource-efficient margin > > 2. On memory-constrained systems (e.g., platforms with only 4GB of > physical memory, which is a common configuration), allocating 4GB > of virtual address space for shadow stack per process/thread can > lead to virtual memory allocation failures when the overcommit mode > is set to OVERCOMMIT_GUESS or OVERCOMMIT_NEVER: > Error: "__vm_enough_memory: not enough memory for the allocation" > > This reduces virtual address space consumption by 50% while maintaining > more than adequate space for return address storage. > > Signed-off-by: Zong Li > --- > arch/riscv/kernel/usercfi.c | 7 ++++--- > 1 file changed, 4 insertions(+), 3 deletions(-) > > diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c > index 6eaa0d94fdfe..4a48dd28d113 100644 > --- a/arch/riscv/kernel/usercfi.c > +++ b/arch/riscv/kernel/usercfi.c > @@ -109,15 +109,16 @@ void set_indir_lp_lock(struct task_struct *task, bool lock) > task->thread_info.user_cfi_state.ufcfi_locked = lock; > } > /* > - * If size is 0, then to be compatible with regular stack we want it to be as big as > - * regular stack. Else PAGE_ALIGN it and return back > + * The shadow stack only stores the return address and not any variables > + * this should be more than sufficient for most applications. > + * Else PAGE_ALIGN it and return back > */ > static unsigned long calc_shstk_size(unsigned long size) > { > if (size) > return PAGE_ALIGN(size); > > - return PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G)); > + return PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK) / 2, SZ_2G)); Use min() instead of min_t(). All the values (before and after the patch) are unsigned. David > } > > /*