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Date:   Tue, 30 Mar 2021 16:22:19 +0200 (CEST)
From:   "Maciej W. Rozycki" <macro@orcam.me.uk>
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>
cc:     YunQiang Su <wzssyqa@gmail.com>,
        YunQiang Su <yunqiang.su@cipunited.com>,
        linux-mips <linux-mips@vger.kernel.org>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        =?UTF-8?Q?Philippe_Mathieu-Daud=C3=A9?= <f4bug@amsat.org>,
        stable@vger.kernel.org
Subject: Re: [PATCH v9] MIPS: force use FR=0 for FPXX binaries
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On Tue, 30 Mar 2021, Thomas Bogendoerfer wrote:

> I want a single go binary, which I can inspect about the FPXX thing and
> see how easy it would be to just patch the binary and make it run without
> this patch.

 I have experimented with this a bit and unfortunately a command like:

$ objcopy --remove-section=.MIPS.abiflags go go-noabiflags

will indeed remove the MIPS ABI Flags section, but will leave an empty 
ABIFLAGS segment behind:

  ABIFLAGS       0x000000 0x00400248 0x00000000 0x00000 0x00000 R   0x4

which the kernel will choke on in `arch_elf_pt_proc':

		if (phdr32->p_filesz < sizeof(abiflags))
			return -EINVAL;

or:

		if (phdr64->p_filesz < sizeof(abiflags))
			return -EINVAL;

which I think is another implementation bug; I think we should silently 
ignore an empty segment like say `readelf -A' does, that is:

		if (!phdr32->p_filesz)
			return 0;

etc.  Sadly there's no option for `objcopy' to explicitly handle segments 
anyhow including to remove empty ones.

 There's the `--update-section' section option for `objcopy' that should 
work, but it requires more effort as an image of a patched .MIPS.abiflags 
section is required (still doable).

  Maciej