Messages in this thread | | | From | Arvind Sankar <> | Date | Sun, 24 May 2020 19:44:02 -0400 | Subject | Re: [PATCH 2/4] x86/boot: Remove runtime relocations from .head.text code |
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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.
> > 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.
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