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AJvYcCWwjdtkJoMTQDAdgSYfI+hM0pJVher4yduPLBRsWS4+Na6ikDGifoAC4IJg7dhWEgApWHDlRBXjSA==@kvack.org X-Gm-Message-State: AOJu0YynuTWf/xwRKxpx0VdZJ+mXcEjsZDSyvA9ryq8fK1Bq1+WLID8G v3RBpn6FbtXtcM9xsq823/Xd6Ow//Il/NC8FThBcuTtbeCtiERz0TU9Bf0jYcO6pHUWo/bRPhIJ 2HwFYjtwcDxWALuzwd9VVDorWm80= X-Google-Smtp-Source: AGHT+IFIKU8LRNjthnXGWB53ZSi2GAcwL6hTYP36TFmTzguRaNKaVZk5srgulC0aAUobh2XIR0/+fYvfyHAf+lqZzfo= X-Received: by 2002:a17:90a:ce81:b0:2c9:1012:b323 with SMTP id 98e67ed59e1d1-2dad50cbcf7mr19247342a91.27.1726001833546; Tue, 10 Sep 2024 13:57:13 -0700 (PDT) MIME-Version: 1.0 References: <20240906051205.530219-1-andrii@kernel.org> <20240906051205.530219-3-andrii@kernel.org> In-Reply-To: From: Andrii Nakryiko Date: Tue, 10 Sep 2024 13:56:59 -0700 Message-ID: Subject: Re: [PATCH 2/2] uprobes: add speculative lockless VMA-to-inode-to-uprobe resolution To: Jann Horn Cc: surenb@google.com, Liam Howlett , Andrii Nakryiko , brauner@kernel.org, linux-trace-kernel@vger.kernel.org, peterz@infradead.org, oleg@redhat.com, rostedt@goodmis.org, mhiramat@kernel.org, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, jolsa@kernel.org, paulmck@kernel.org, willy@infradead.org, akpm@linux-foundation.org, linux-mm@kvack.org, mjguzik@gmail.com, Miklos Szeredi , Amir Goldstein , linux-fsdevel Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam03 X-Rspam-User: X-Rspamd-Queue-Id: 0E21F16002E X-Stat-Signature: 63occq8co9j5xthy8wsrrzhyakgygepo X-HE-Tag: 1726001834-449204 X-HE-Meta: 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 DeNt4KS0 I4+F3g0V+fF0Cg3Kt7ODhH/ok6x0Ih6hXocp4a9xvs/YynUHgAuOb6Xh1VZfrzOzd1o22BjQko/kRA6mZJ8P6y9dcC0raXfoP1tyD0OtlGYwBvf+OnXWyTDuLQnJlM0lyvfnOC51j7fNYmgY7by9VnGVbUGxgydhA3a7zh0vIygWppfauuVt7uEd3dAi9B25fn0KYL4228z6zjw/ncoHf6lTiOpo6GW9OGABfKwKEvvyJDwGL0qc71ofyGwS0zp687mvJQcgHDuTNRgZgaWUuKxPNGBaVVGC/YJAECr3yzJaMIbXZm7sIV1dIS77X7im4jX6mkJiwWQVQGCaOT7PF5OG9gna3IrgRQS25Qq/C9rlVW6XpEEF+bT2QxI0z2z1L7JqHrskx+t0MKV8KAP8F1PeA6Gh7+K9bXnzlJfeGdpXtFWo= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, 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 Tue, Sep 10, 2024 at 8:39=E2=80=AFAM Jann Horn wrote: > > On Mon, Sep 9, 2024 at 11:29=E2=80=AFPM Andrii Nakryiko > wrote: > > On Mon, Sep 9, 2024 at 6:13=E2=80=AFAM Jann Horn wro= te: > > > On Fri, Sep 6, 2024 at 7:12=E2=80=AFAM Andrii Nakryiko wrote: > > > > +static struct uprobe *find_active_uprobe_speculative(unsigned long= bp_vaddr) > > > > +{ > > > > + const vm_flags_t flags =3D VM_HUGETLB | VM_MAYEXEC | VM_MAY= SHARE; > > > > + struct mm_struct *mm =3D current->mm; > > > > + struct uprobe *uprobe; > > > > + struct vm_area_struct *vma; > > > > + struct file *vm_file; > > > > + struct inode *vm_inode; > > > > + unsigned long vm_pgoff, vm_start; > > > > + int seq; > > > > + loff_t offset; > > > > + > > > > + if (!mmap_lock_speculation_start(mm, &seq)) > > > > + return NULL; > > > > + > > > > + rcu_read_lock(); > > > > + > > > > + vma =3D vma_lookup(mm, bp_vaddr); > > > > + if (!vma) > > > > + goto bail; > > > > + > > > > + vm_file =3D data_race(vma->vm_file); > > > > > > A plain "data_race()" says "I'm fine with this load tearing", but > > > you're relying on this load not tearing (since you access the vm_file > > > pointer below). > > > You're also relying on the "struct file" that vma->vm_file points to > > > being populated at this point, which means you need CONSUME semantics > > > here, which READ_ONCE() will give you, and something like RELEASE > > > semantics on any pairing store that populates vma->vm_file, which > > > means they'd all have to become something like smp_store_release()). > > > > vma->vm_file should be set in VMA before it is installed and is never > > modified afterwards, isn't that the case? So maybe no extra barrier > > are needed and READ_ONCE() would be enough. > > Ah, right, I'm not sure what I was thinking there. > > I... guess you only _really_ need the READ_ONCE() if something can > actually ever change the ->vm_file pointer, otherwise just a plain > load with no annotation whatsoever would be good enough? I'm fairly yep, probably, I was just trying to be cautious :) > sure nothing can ever change the ->vm_file pointer of a live VMA, and > I think _currently_ it looks like nothing will NULL out the ->vm_file > pointer on free either... though that last part is probably not > something you should rely on... This seems to be rather important, but similarly to how vm_file can't be modified, it seems reasonable to assume that it won't be set to NULL (it's a modification to set it to a new NULL value, isn't it?). I mean, we can probably just add a NULL check and rely on the atomicity of setting a pointer, so not a big deal, but seems like a pretty reasonable assumption to make.