From: liuzhensong <liuzhensong@loongson.cn>
To: Xi Ruoyao <xry111@xry111.site>, binutils@sourceware.org
Cc: WANG Xuerui <i.swmail@xen0n.name>, chenglulu@loongson.cn
Subject: Re: [PATCH] LoongArch: Use copy relocation for %pc_lo12 against external symbol
Date: Thu, 1 Sep 2022 09:38:46 +0800 [thread overview]
Message-ID: <3b43c733-6011-8c7c-4a1e-3faa9f91f711@loongson.cn> (raw)
Message-ID: <20220901013846.SfjWQdSfsD84gJbn26f79lcS1dWuoBLMh3kt8pl0w00@z> (raw)
In-Reply-To: <bb99494e9d2f646169510ff3d0c8924966958354.camel@xry111.site>
[-- Attachment #1: Type: text/plain, Size: 2043 bytes --]
在 2022/8/31 下午9:41, Xi Ruoyao 写道:
> Some self review:
>
> On Wed, 2022-08-31 at 21:22 +0800, Xi Ruoyao wrote:
>> - /* For pcalau12i + jirl. */
>> - h->needs_plt = 1;
>> - if (h->plt.refcount < 0)
>> - h->plt.refcount = 0;
>> - h->plt.refcount++;
>> + /* Check if it's a jirl instruction. */
>> + bfd_byte *contents = elf_section_data (sec)->this_hdr.contents;
>> + uint32_t insn = 0;
>> +
>> + if (contents || bfd_malloc_and_get_section (abfd, sec, &contents))
>> + memcpy(&insn, contents + rel->r_offset, sizeof(insn));
>> +
>> + if ((insn & 0xfc000000) == 0x4c000000)
> This is tricky. But in commit 42bd525 we already started to rely on
> undocumented %pc_{hi20,lo12} behavior: if you just apply them "as
> documented" to the pcalau12i/jirl pairs the result will be absolutely
> wrong. And 42bd525 behavior is not fully correct: if you just write
>
> pcalau12i $t0, %pc_hi20(data)
> ld.d, $t0, $t0, %pc_lo12(data)
Do you have a test case?
>
> With 42bd525, the BFD linker generates a "PLT for data" (absolutely
> nonsense), and destroys the ld.d instruction. All of these buggy
> behavior happens silently, the user will only find something wrong when
> the linked program crashes.
>
> I'm not sure if checking JIRL (i. e. the behavior of a relocation now
> depends on the instruction where it's applied) is a good idea. Maybe we
> should use the following instead of pcalau12i/jirl:
>
> pcaddu18i $t0, %b16_hi20(func)
> jirl $ra, $t0, %b16(func)
>
> ("b16_hi20" is a hypothetical thing here.) This will make things less
> tricky, and also expand the range to 128 GiB.
It doesn't make sense for only "pcaddu18i + jirl" to access 128G.What we
need is a jump that can access ±2G, just like any other pc-relative instructions can access ±2G.
next prev parent reply other threads:[~2022-09-01 1:38 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-31 13:22 Xi Ruoyao
2022-08-31 13:41 ` Xi Ruoyao
2022-09-01 1:38 ` liuzhensong [this message]
2022-09-01 1:38 ` liuzhensong
2022-09-01 2:12 ` Xi Ruoyao
2022-09-01 2:31 ` liuzhensong
2022-09-01 2:31 ` liuzhensong
2022-09-01 1:27 ` liuzhensong
2022-09-01 1:27 ` liuzhensong
2022-09-01 1:41 ` Xi Ruoyao
2022-09-01 2:09 ` liuzhensong
2022-09-01 7:42 ` Fangrui Song
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