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[163.53.146.23]) by smtp.gmail.com with ESMTPSA id ada2fe7eead31-78a47ae301fsm8384438137.7.2026.09.08.04.57.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 08 Sep 2026 04:57:10 -0700 (PDT) Date: Tue, 8 Sep 2026 21:56:53 +1000 From: Matt Bobrowski To: Anastasios Papagiannis Cc: bpf@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, david@kernel.org, akpm@linux-foundation.org, andrii@kernel.org, ast@kernel.org, brauner@kernel.org, daniel@iogearbox.net, eddyz87@gmail.com, kpsingh@kernel.org, ljs@kernel.org, memxor@gmail.com, song@kernel.org, sun.jian.kdev@gmail.com, utilityemal77@gmail.com, viro@zeniv.linux.org.uk Subject: Re: [PATCH bpf-next v5 3/7] bpf: Add user memory access kfuncs for mm_struct Message-ID: References: <20260907165220.52431-1-tasos.papagiannnis@gmail.com> <20260907165220.52431-4-tasos.papagiannnis@gmail.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260907165220.52431-4-tasos.papagiannnis@gmail.com> On Mon, Sep 07, 2026 at 07:52:16PM +0300, Anastasios Papagiannis wrote: > On CONFIG_MMU kernels, when security_bprm_check() runs, the argument and > environment strings for the exec have been copied into bprm->mm. The new > address space is not associated with a task_struct until exec_mmap(), so > existing BPF user memory helpers cannot access it. > > Add bpf_copy_from_user_mm() and bpf_copy_from_user_mm_str() kfuncs. Both > take a struct mm_struct pointer directly, allowing callers to access > trusted address spaces that are not associated with a task_struct. > > bpf_copy_from_user_mm() has similar semantics to > bpf_copy_from_user_task(). bpf_copy_from_user_mm_str() copies one > NUL-terminated string and returns its size including the NUL terminator. > It accepts BPF_F_PAD_ZEROS to clear unused destination bytes on success. > > Refactor bpf_copy_from_user_task() and bpf_copy_from_user_task_str() to > acquire the task's mm with get_task_mm() and delegate to the corresponding > mm-based implementations. No behavior change is intended for the existing > task-based interfaces. > > Register both new kfuncs and mark them KF_SLEEPABLE because accessing a > remote address space can fault. > > Signed-off-by: Anastasios Papagiannis A couple nits here. > --- > kernel/bpf/helpers.c | 142 ++++++++++++++++++++++++++++++++++--------- > 1 file changed, 113 insertions(+), 29 deletions(-) > > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c > index b3cc5c8fc875..d3c564437ad0 100644 > --- a/kernel/bpf/helpers.c > +++ b/kernel/bpf/helpers.c > @@ -32,6 +32,10 @@ > > #include "../../lib/kstrtox.h" > > +__bpf_kfunc int bpf_copy_from_user_mm(void *dst, u32 dst__sz, > + const void __user *unsafe_ptr__ign, > + struct mm_struct *mm, u64 flags); > + This shouldn't be needed. kfuncs are prototype-free by design, which is what the -Wmissing-prototypes suppression in __bpf_kfunc_start_defs() is for, and __bpf_kfuncs expands to __used __retain noinline, which are definition attributes. Additionally, I feel as though any shared pieces of infrastructure amongst these kfuncs/helpers should live within their own static internal helpers (__bpf_copy_from_user_mm() and __bpf_copy_from_user_mm_str() or something like that. Callers should also guarantee that a live mm is being passed into each respective helper). That also lets the compiler inline them, rather than being blocked by noinline on both ends. > /* If kernel subsystem is allowing eBPF programs to call this function, > * inside its own verifier_ops->get_func_proto() callback it should return > * bpf_map_lookup_elem_proto, so that verifier can properly check the arguments > @@ -682,22 +686,15 @@ const struct bpf_func_proto bpf_copy_from_user_proto = { > BPF_CALL_5(bpf_copy_from_user_task, void *, dst, u32, size, > const void __user *, user_ptr, struct task_struct *, tsk, u64, flags) > { > + struct mm_struct *mm; > int ret; > > - /* flags is not used yet */ > - if (unlikely(flags)) > - return -EINVAL; > - > - if (unlikely(!size)) > - return 0; > - > - ret = access_process_vm(tsk, (unsigned long)user_ptr, dst, size, 0); > - if (ret == size) > - return 0; > + mm = get_task_mm(tsk); I'd keep the flags and size checks before get_task_mm(). Otherwise a bad flags value or size results in us taking a task_lock() and an mm reference before returning -EINVAL. So I'd argue that you changing these semantics alone doesn't allow you to uphold your statement around there being "no behavior change". > + ret = bpf_copy_from_user_mm(dst, size, user_ptr, mm, flags); > + if (mm) > + mmput(mm); > > - memset(dst, 0, size); > - /* Return -EFAULT for partial read */ > - return ret < 0 ? ret : -EFAULT; > + return ret; > } > > const struct bpf_func_proto bpf_copy_from_user_task_proto = { > @@ -3658,6 +3655,100 @@ __bpf_kfunc int bpf_copy_from_user_str(void *dst, u32 dst__sz, const void __user > return ret + 1; > } > > +/** > + * bpf_copy_from_user_mm() - Copy data from an address space > + * @dst: Destination address, in kernel space > + * @dst__sz: Number of bytes to copy > + * @unsafe_ptr__ign: Source address in the address space > + * @mm: Address space to copy from > + * @flags: Reserved for future use; must be zero > + * > + * Copies data from the user address space associated with @mm. The destination > + * is zeroed if an attempted copy cannot be completed in full. Unsupported > + * flags return -EINVAL without modifying @dst. > + * > + * Return: 0 on success, -EINVAL if @flags is non-zero, or -EFAULT if the copy > + * fails or is partial. > + */ > +__bpf_kfunc int bpf_copy_from_user_mm(void *dst, u32 dst__sz, > + const void __user *unsafe_ptr__ign, > + struct mm_struct *mm, u64 flags) > +{ > + int ret; > + > + if (unlikely(flags)) > + return -EINVAL; > + > + if (unlikely(!dst__sz)) > + return 0; > + > + if (unlikely(!mm)) { > + memset(dst, 0, dst__sz); > + return -EFAULT; > + } > + > + ret = access_remote_vm(mm, (unsigned long)unsafe_ptr__ign, > + dst, dst__sz, 0); > + if (ret == dst__sz) > + return 0; > + > + memset(dst, 0, dst__sz); > + return ret < 0 ? ret : -EFAULT; > +} > + > +/** > + * bpf_copy_from_user_mm_str() - Copy a string from an address space > + * @dst: Destination address, in kernel space. This buffer must be > + * at least @dst__sz bytes long > + * @dst__sz: Maximum number of bytes to copy, including the trailing NUL > + * @unsafe_ptr__ign: Source address in the address space > + * @mm: Address space to copy from > + * @flags: The only supported flag is BPF_F_PAD_ZEROS > + * > + * Copies a NUL-terminated string from the user address space associated with > + * @mm. If the string is too long, @dst is still NUL-terminated unless @dst__sz > + * is zero. > + * > + * If the flags are valid and BPF_F_PAD_ZEROS is set, the unused portion of > + * @dst is cleared on success and all of @dst is cleared on a copy failure. > + * Unsupported flags return -EINVAL without modifying @dst. > + * > + * Return: The number of copied bytes including the NUL terminator on success, > + * or a negative error code on failure. > + */ > +__bpf_kfunc int bpf_copy_from_user_mm_str(void *dst, u32 dst__sz, > + const void __user *unsafe_ptr__ign, > + struct mm_struct *mm, u64 flags) > +{ > + int ret; > + > + if (unlikely(flags & ~BPF_F_PAD_ZEROS)) > + return -EINVAL; > + > + if (unlikely(dst__sz == 0)) > + return 0; > + > + if (unlikely(!mm)) { > + if (flags & BPF_F_PAD_ZEROS) > + memset(dst, 0, dst__sz); > + else > + *(char *)dst = '\0'; > + return -EFAULT; > + } > + > + ret = copy_remote_mm_str(mm, (unsigned long)unsafe_ptr__ign, dst, dst__sz, 0); > + if (ret < 0) { > + if (flags & BPF_F_PAD_ZEROS) > + memset(dst, 0, dst__sz); > + return ret; > + } > + > + if (flags & BPF_F_PAD_ZEROS) > + memset(dst + ret, 0, dst__sz - ret); > + > + return ret + 1; > +} > + > /** > * bpf_copy_from_user_task_str() - Copy a string from an task's address space > * @dst: Destination address, in kernel space. This buffer must be > @@ -3681,25 +3772,16 @@ __bpf_kfunc int bpf_copy_from_user_task_str(void *dst, u32 dst__sz, > const void __user *unsafe_ptr__ign, > struct task_struct *tsk, u64 flags) > { > + struct mm_struct *mm; > int ret; > > - if (unlikely(flags & ~BPF_F_PAD_ZEROS)) > - return -EINVAL; > - > - if (unlikely(dst__sz == 0)) > - return 0; > + mm = get_task_mm(tsk); > + ret = bpf_copy_from_user_mm_str(dst, dst__sz, unsafe_ptr__ign, > + mm, flags); > + if (mm) > + mmput(mm); > > - ret = copy_remote_vm_str(tsk, (unsigned long)unsafe_ptr__ign, dst, dst__sz, 0); > - if (ret < 0) { > - if (flags & BPF_F_PAD_ZEROS) > - memset(dst, 0, dst__sz); > - return ret; > - } > - > - if (flags & BPF_F_PAD_ZEROS) > - memset(dst + ret, 0, dst__sz - ret); > - > - return ret + 1; > + return ret; > } > > /* Keep unsigned long in prototype so that kfunc is usable when emitted to > @@ -4924,6 +5006,8 @@ BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW) > BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL) > BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY) > BTF_ID_FLAGS(func, bpf_copy_from_user_str, KF_SLEEPABLE) > +BTF_ID_FLAGS(func, bpf_copy_from_user_mm, KF_SLEEPABLE) > +BTF_ID_FLAGS(func, bpf_copy_from_user_mm_str, KF_SLEEPABLE) > BTF_ID_FLAGS(func, bpf_copy_from_user_task_str, KF_SLEEPABLE) > BTF_ID_FLAGS(func, bpf_get_kmem_cache) > BTF_ID_FLAGS(func, bpf_iter_kmem_cache_new, KF_ITER_NEW | KF_SLEEPABLE) > -- > 2.55.0 >