From 8819b23615c9176bbb7d2a0a5036bab616010048 Mon Sep 17 00:00:00 2001 From: Mark Harmstone Date: Sat, 31 Dec 2022 20:55:46 +0000 Subject: [PATCH] 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$4 entry) to be aligned to 8 bytes. Without this you get IMAGE_REL_ARM64_PAGEOFFSET_12L overflow errors. --- ld/scripttempl/pep.sc | 1 + 1 file changed, 1 insertion(+) diff --git a/ld/scripttempl/pep.sc b/ld/scripttempl/pep.sc index 1b0f0df89ae..df1f4116ca4 100644 --- a/ld/scripttempl/pep.sc +++ b/ld/scripttempl/pep.sc @@ -34,6 +34,7 @@ if test "${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=' -- 2.30.2