[binutils-gdb] Avoid unaligned pointer reads in PEP .idata section

[Mark Harmstone]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=8819b23615c9176bbb7d2a0a5036bab616010048

commit 8819b23615c9176bbb7d2a0a5036bab616010048
Author: Mark Harmstone <mark@harmstone.com>
Date:   Sat Dec 31 20:55:46 2022 +0000

    Avoid unaligned pointer reads in PEP .idata section
    
    This is something I discovered when working on aarch64, though it's
    relevant to x86_64 too.
    
    The PE32+ imports are located in the .idata section, which starts off
    with a 20-byte structure for each DLL, containing offsets into the rest
    of the section. This is the Import Directory Table in
    https://learn.microsoft.com/en-us/windows/win32/debug/pe-format, which
    is a concatenation of the .idata$2 sections. This is then followed by an
    20 zero bytes generated by the linker script, which calls this .idata$3.
    
    After this comes the .idata$4 entries for each function, which the
    loader overwrites with the function pointers. Because there's no padding
    between .idata$3 and .idata$4, this means that if there's an even number
    of DLLs, the function pointers won't be aligned on an 8-byte boundary.
    
    Misaligned reads are slower on x86_64, but this is more important on
    aarch64, as the e.g. `ldr x0, [x0, :lo12:__imp__func]` the compiler
    might generate requires __imp__func (the .idata$4 entry) to be aligned
    to 8 bytes. Without this you get IMAGE_REL_ARM64_PAGEOFFSET_12L overflow
    errors.

Diff:
---
 ld/scripttempl/pep.sc | 1 +
 1 file changed, 1 insertion(+)

diff --git a/ld/scripttempl/pep.sc b/ld/scripttempl/pep.sc
index 1b0f0df89ae..df1f4116ca4 100644
--- a/ld/scripttempl/pep.sc
+++ b/ld/scripttempl/pep.sc
@@ -34,6 +34,7 @@ if test "${RELOCATING}"; then
     KEEP (SORT(*)(.idata$3))
     /* These zeroes mark the end of the import list.  */
     LONG (0); LONG (0); LONG (0); LONG (0); LONG (0);
+    . = ALIGN(8);
     KEEP (SORT(*)(.idata$4))'
   R_IDATA5='SORT(*)(.idata$5)'
   R_IDATA67='