Re: [PATCH] LoongArch: Use copy relocation for %pc_lo12 against external symbol

[Xi Ruoyao <xry111@xry111.site>]

On Thu, 2022-09-01 at 09:38 +0800, liuzhensong wrote:
> > 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?

$ cat t2.s   
.text
.align 2
.type x, @function
.global x
x:
	pcalau12i	$a0, %pc_hi20(data)
	ld.d		$a0, $a0, %pc_lo12(data)
	jr		$ra
$ gcc t2.s -c
$ ./ld/ld-new t2.o -shared
$ objdump -d | grep -A50 data
0000000000000210 <data@plt>:
 210:	1c00010f 	pcaddu12i   	$t3, 8(0x8)
 214:	28f801ef 	ld.d        	$t3, $t3, -512(0xe00)
 218:	4c0001ed 	jirl        	$t1, $t3, 0
 21c:	03400000 	andi        	$zero, $zero, 0x0

Disassembly of section .text:

0000000000000220 <x>:
 220:	1a000004 	pcalau12i   	$a0, 0
 224:	28c84084 	ld.d        	$a0, $a0, 528(0x210)
 228:	4c000020 	jirl        	$zero, $ra, 0

i.e.  Instead of reporting an error like "cannot create a runtime
relocation against external symbol 'data'", the linker silently produces
a PLT (nonsense: can you use a PLT for data?) and load two instructions
from the PLT into the register (also nonsense).  So if someone mistypes
"la.local" where "la.global" should be used (it's just a simple
programming mistake, and it's likely to happen in the practice!), the
linking will "succeeds" silently.  Then the program blows up at runtime.

> 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.

The point is, if we interpret %pc_lo12 "as it's documented":

    "(*(uint32_t *) PC) [21 ... 10] = (S+A) [11 ... 0]"

it will be absolutely wrong for a jirl instruction.  You may update the
doc to say something like "if R_LARCH_PCALA_LO12 is applied to a jirl
instruction, a PLT entry will be created and blah blah". But again I'm
not sure about if "the behavior of a relocation depends on the
instruction for which it's applied" is a good idea.

<rant>We are using highly imprecise descriptions for PCALA-style
relocations in ELF psABI, despite I've disagreed in the review.  Now if
someone wants to know "how this relocation will *really* behave", he/she
will need to read BFD code.  PCALAU12I instruction itself is already
puzzling enough (comparing with PCADDU12I, which behaves more "normal"),
now the doc just makes it more puzzling.</rant>
-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University