From: Mark Harmstone Date: Sat, 31 Dec 2022 20:55:46 +0000 (+0000) Subject: Avoid unaligned pointer reads in PEP .idata section X-Git-Tag: binutils-2_40~36 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=2ebe33ffd1e1208a16178364ad0f2ea859f16685;p=thirdparty%2Fbinutils-gdb.git 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$4 entry) to be aligned to 8 bytes. Without this you get IMAGE_REL_ARM64_PAGEOFFSET_12L overflow errors. --- diff --git a/ld/scripttempl/pep.sc b/ld/scripttempl/pep.sc index cb404eb070a..8632955f08f 100644 --- a/ld/scripttempl/pep.sc +++ b/ld/scripttempl/pep.sc @@ -34,6 +34,7 @@ if test "${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='