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Message-ID: <b39b79d6-4fe2-f887-dc7e-03b62d6eb3e4@fb.com> Date: Thu, 11 May 2017 16:10:35 -0700 From: Alexei Starovoitov <ast@...com> To: David Miller <davem@...emloft.net> CC: <daniel@...earbox.net>, <netdev@...r.kernel.org> Subject: Re: BPF relocations On 5/11/17 12:31 PM, David Miller wrote: > > I haven't done more work on bintuils BPF support because we > need to figure out exactly what to do with relocations. So > I've been trying to spend time thinking about this. > > As far as I can tell the 64-bit BPF relocation llvm uses > is used in two situations: > > 1) 64-bit relocations against data > > 2) 64-bit relocations against ldimm64 instructions > > If this is true it's a very bad decision that has ramifications for us > right now. > > One must always explicitly define relocations as being against data or > instruction fields. You cannot use the same relocation for both kinds > of transformations, somehow trying to figure out what to do > "contextually". That doesn't work. why it doesn't work? as far as i can see x86 doesn't care where the relo applies. afaik relocations are divided into absolute, pc relative and pic relative and it doesn't matter whether they're against .text, .eh_frame or .debug_* sections. We have just two so far: absolute 32-bit and absolute 64-bit relocation. I don't see what we would use pc-relative relo for. If I remember correctly, the x64 and other cpus use pc-relative for 'call foo' insns since this is how those instructions work. We don't have such calls yet.
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