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SubjectRe: [PATCH 2/4] x86/boot: Remove runtime relocations from .head.text code

On 2020-05-24, Arvind Sankar wrote:
>On Sun, May 24, 2020 at 03:53:59PM -0700, Fangrui Song wrote:
>> On 2020-05-24, Arvind Sankar wrote:
>> >The assembly code in head_{32,64}.S, while meant to be
>> >position-independent, generates run-time relocations because it uses
>> >instructions such as
>> > leal gdt(%edx), %eax
>> >which make the assembler and linker think that the code is using %edx as
>> >an index into gdt, and hence gdt needs to be relocated to its run-time
>> >address.
>> >
>> >With the BFD linker, this generates a warning during the build:
>> > LD arch/x86/boot/compressed/vmlinux
>> >ld: arch/x86/boot/compressed/head_32.o: warning: relocation in read-only section `.head.text'
>> >ld: warning: creating a DT_TEXTREL in object
>>
>> Interesting. How does the build generate a warning by default?
>> Do you use Gentoo Linux which appears to ship a --warn-shared-textrel
>> enabled-by-default patch? (https://bugs.gentoo.org/700488)
>
>Ah, yes I am using gentoo. I didn't realize that was a distro
>modification.
>
>> >+
>> >+/*
>> >+ * This macro gives the link address of X. It's the same as X, since startup_32
>> >+ * has link address 0, but defining it this way tells the assembler/linker that
>> >+ * we want the link address, and not the run-time address of X. This prevents
>> >+ * the linker from creating a run-time relocation entry for this reference.
>> >+ * The macro should be used as a displacement with a base register containing
>> >+ * the run-time address of startup_32 [i.e. la(X)(%reg)], or as an
>> >+ * immediate [$ la(X)].
>> >+ *
>> >+ * This macro can only be used from within the .head.text section, since the
>> >+ * expression requires startup_32 to be in the same section as the code being
>> >+ * assembled.
>> >+ */
>> >+#define la(X) ((X) - startup_32)
>> >+
>>
>> IIRC, %ebp contains the address of startup_32. la(X) references X
>> relative to startup_32. The fixup (in GNU as and clang integrated
>> assembler's term) is a constant which is resolved by the assembler.
>>
>> There is no R_386_32 or R_386_PC32 for the linker to resolve.
>
>This is incorrect (or maybe I'm not understanding you correctly). X is a
>symbol whose final location relative to startup_32 is in most cases not
>known to the assembler (there are a couple of cases where X is a label
>within .head.text which do get completely resolved by the assembler).
>
>For example, taking the instruction loading the gdt address, this is
>what we get from the assembler:
> lea la(gdt)(%ebp), %eax
>becomes in the object file:
> 11: 8d 85 00 00 00 00 lea 0x0(%ebp),%eax
> 13: R_X86_64_PC32 .data+0x23
>or a cleaner example using a global symbol:
> subl la(image_offset)(%ebp), %ebx
>becomes
> 41: 2b 9d 00 00 00 00 sub 0x0(%ebp),%ebx
> 43: R_X86_64_PC32 image_offset+0x43
>
>So in general you get PC32 relocations, with the addend being set by the
>assembler to .-startup_32, modulo the adjustment for where within the
>instruction the displacement needs to be. These relocations are resolved
>by the static linker to produce constants in the final executable.
>

You are right. I am not familiar with the code and only looked at 1b.

Just preprocessed head_64.S and verified many target symbols are in
.data and .pgtable The assembler converts an expression `foo - symbol_defined_in_same_section`
to be `foo - . + offset` which can be encoded as an R_X86_64_PC32 (or
resolved the fixup if it is a constant, e.g. `1b - startup_32`)

>>
>> Not very sure stating that "since startup_32 has link address 0" is very
>> appropriate here (probably because I did't see the term "link address"
>> before). If my understanding above is correct, I think you can just
>> reword the comment to express that X is referenced relative to
>> startup_32, which is stored in %ebp.
>>
>
>Yeah, the more standard term is virtual address/VMA, but that sounds
>confusing to me with PIE code since the _actual_ virtual address at
>which this code is going to run isn't 0, that's just the address assumed
>for linking. I can reword it to avoid referencing "link address" but
>then it's not obvious why the macro is named "la" :) I'm open to
>suggestions on a better name, I could use offset but that's a bit
>long-winded. I could just use vma() if nobody else finds it confusing.
>
>Thanks.

With your approach, the important property is that "the distance between
startup_32 and the target symbol is a constant", not that "startup_32
has link address 0". `ra`, `rva`, `rvma` or any other term which expresses "relative"
should work. Hope someone can come up with a good suggestion:)

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