Messages in this thread | | | Date | Sun, 24 May 2020 17:55:11 -0700 | From | Fangrui Song <> | Subject | Re: [PATCH 2/4] x86/boot: Remove runtime relocations from .head.text code |
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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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