From: Mark Harmstone <mark@harmstone.com>
To: binutils@sourceware.org, zac.walker@linaro.org
Cc: Mark Harmstone <mark@harmstone.com>
Subject: [PATCH] Avoid unaligned pointer reads in PEP .idata section
Date: Sat, 31 Dec 2022 20:55:46 +0000 [thread overview]
Message-ID: <20221231205546.14330-1-mark@harmstone.com> (raw)
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$4 entry) to be aligned
to 8 bytes. Without this you get IMAGE_REL_ARM64_PAGEOFFSET_12L overflow
errors.
---
ld/scripttempl/pep.sc | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/ld/scripttempl/pep.sc b/ld/scripttempl/pep.sc
index f8fa74364aa..2091e8df0fd 100644
--- a/ld/scripttempl/pep.sc
+++ b/ld/scripttempl/pep.sc
@@ -34,6 +34,7 @@ if test "${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='
--
2.37.4
next reply other threads:[~2022-12-31 20:55 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-31 20:55 Mark Harmstone [this message]
2023-01-03 11:50 ` Nick Clifton
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