Patch "x86: Treat R_X86_64_PLT32 as R_X86_64_PC32" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

    x86: Treat R_X86_64_PLT32 as R_X86_64_PC32

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     x86-treat-r_x86_64_plt32-as-r_x86_64_pc32.patch

and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "H.J. Lu" <[email protected]>
Date: Wed, 7 Feb 2018 14:20:09 -0800
Subject: x86: Treat R_X86_64_PLT32 as R_X86_64_PC32

From: H.J. Lu <[email protected]>

commit b21ebf2fb4cde1618915a97cc773e287ff49173e upstream.

On i386, there are 2 types of PLTs, PIC and non-PIC. PIE and shared objects must use PIC PLT. To use PIC PLT, you need to load _GLOBAL_OFFSET_TABLE_ into EBX first. There is no need for that on x86-64 since x86-64 uses PC-relative PLT.

On x86-64, for 32-bit PC-relative branches, we can generate PLT32 relocation, instead of PC32 relocation, which can also be used as a marker for 32-bit PC-relative branches. Linker can always reduce
PLT32 relocation to PC32 if function is defined locally. Local
functions should use PC32 relocation. As far as Linux kernel is concerned, R_X86_64_PLT32 can be treated the same as R_X86_64_PC32 since Linux kernel doesn't use PLT.

R_X86_64_PLT32 for 32-bit PC-relative branches has been enabled in binutils master branch which will become binutils 2.31.

[ hjl is working on having better documentation on this all, but a few
more notes from him:

"PLT32 relocation is used as marker for PC-relative branches. Because
of EBX, it looks odd to generate PLT32 relocation on i386 when EBX doesn't have GOT.

As for symbol resolution, PLT32 and PC32 relocations are almost interchangeable. But when linker sees PLT32 relocation against a protected symbol, it can resolved locally at link-time since it is used on a branch instruction. Linker can't do that for PC32 relocation"

but for the kernel use, the two are basically the same, and this commit gets things building and working with the current binutils
master - Linus ]

---
 arch/x86/kernel/machine_kexec_64.c |    1 +
 arch/x86/kernel/module.c           |    1 +
 arch/x86/tools/relocs.c            |    3 +++
 3 files changed, 5 insertions(+)

@@ -524,6 +524,7 @@ int arch_kexec_apply_relocations_add(con
 				goto overflow;
 			break;
 		case R_X86_64_PC32:
+		case R_X86_64_PLT32:
 			value -= (u64)address;
 			*(u32 *)location = value;
 			break;

@@ -190,6 +190,7 @@ int apply_relocate_add(Elf64_Shdr *sechd
 				goto overflow;
 			break;
 		case R_X86_64_PC32:
+		case R_X86_64_PLT32:
 			if (*(u32 *)loc != 0)
 				goto invalid_relocation;
 			val -= (u64)loc;

@@ -769,9 +769,12 @@ static int do_reloc64(struct section *se
 		break;
 
 	case R_X86_64_PC32:
+	case R_X86_64_PLT32:
 		/*
 		 * PC relative relocations don't need to be adjusted unless
 		 * referencing a percpu symbol.
+		 *
+		 * NB: R_X86_64_PLT32 can be treated as R_X86_64_PC32.
 		 */
 		if (is_percpu_sym(sym, symname))
 			add_reloc(&relocs32neg, offset);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/x86-treat-r_x86_64_plt32-as-r_x86_64_pc32.patch