From: Peter Zijlstra <peterz@infradead.org>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
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Subject: Re: [PATCH 1/3] module: Rename module_alloc() to text_alloc() and move to kernel proper
Date: Tue, 14 Jul 2020 13:45:20 +0200 [thread overview]
Message-ID: <20200714114520.GW10769@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200714103333.GB1551@shell.armlinux.org.uk>
On Tue, Jul 14, 2020 at 11:33:33AM +0100, Russell King - ARM Linux admin wrote:
> For 32-bit ARM, our bpf code uses "blx/bx" (or equivalent code
> sequences) rather than encoding a "bl" or "b", so BPF there doesn't
> care where the executable memory is mapped, and doesn't need any
> PLTs. Given that, should bpf always allocate from the vmalloc()
> region to preserve the module space for modules?
Ah, okay, then I suspect arm64 does something similar there. Thanks!
> I'm more concerned about ftrace though, but only because I don't
> have the understanding of that subsystem to really say whether there
> are any side effects from having the allocations potentially be out
> of range of a "bl" or "b" instruction.
>
> If ftrace jumps both to and from the allocated page using a "load
> address to register, branch to register" approach like BPF does, then
> ftrace should be safe - and again, raises the issue that maybe it
> should always come from vmalloc() space.
I think the problem with ftrace is patching multiple instruction;
because it sounds like you'd need something to load the absolute address
in a register and then jump to that. And where it's relatively easy to
replace a single instruction, replace multiple instructions gets real
tricky real quick.
Which then leads to you being stuck with that 26bit displacement, IIRC.
next prev parent reply other threads:[~2020-07-14 11:55 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-13 18:19 [PATCH 1/3] module: Rename module_alloc() to text_alloc() and move to kernel proper Jarkko Sakkinen
2020-07-13 18:34 ` Russell King - ARM Linux admin
2020-07-14 9:49 ` Jarkko Sakkinen
2020-07-14 9:53 ` Russell King - ARM Linux admin
2020-07-14 11:43 ` Jarkko Sakkinen
2020-07-13 19:49 ` Ard Biesheuvel
2020-07-14 2:04 ` Steven Rostedt
2020-07-14 6:35 ` Ard Biesheuvel
2020-07-14 9:52 ` Jarkko Sakkinen
2020-07-14 10:17 ` Ard Biesheuvel
2020-07-14 10:33 ` Russell King - ARM Linux admin
2020-07-14 11:45 ` Peter Zijlstra [this message]
2020-07-14 12:11 ` Jarkko Sakkinen
2020-07-14 11:55 ` Jarkko Sakkinen
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