This is a note to let you know that I've just added the patch titled
module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for undefined symbols
to the 4.4-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:
module-ignore-_global_offset_table_-when-warning-for-undefined-symbols.patch
and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From ebfac7b778fac8b0e8e92ec91d0b055f046b4604 Mon Sep 17 00:00:00 2001
From: Fangrui Song <maskray@xxxxxxxxxx>
Date: Fri, 15 Jan 2021 11:52:22 -0800
Subject: module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for undefined symbols
From: Fangrui Song <maskray@xxxxxxxxxx>
commit ebfac7b778fac8b0e8e92ec91d0b055f046b4604 upstream.
clang-12 -fno-pic (since
https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)
can emit `call __stack_chk_fail@PLT` instead of `call __stack_chk_fail`
on x86. The two forms should have identical behaviors on x86-64 but the
former causes GNU as<2.37 to produce an unreferenced undefined symbol
_GLOBAL_OFFSET_TABLE_.
(On x86-32, there is an R_386_PC32 vs R_386_PLT32 difference but the
linker behavior is identical as far as Linux kernel is concerned.)
Simply ignore _GLOBAL_OFFSET_TABLE_ for now, like what
scripts/mod/modpost.c:ignore_undef_symbol does. This also fixes the
problem for gcc/clang -fpie and -fpic, which may emit `call foo@PLT` for
external function calls on x86.
Note: ld -z defs and dynamic loaders do not error for unreferenced
undefined symbols so the module loader is reading too much. If we ever
need to ignore more symbols, the code should be refactored to ignore
unreferenced symbols.
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://github.com/ClangBuiltLinux/linux/issues/1250
Link: https://sourceware.org/bugzilla/show_bug.cgi?id=27178
Reported-by: Marco Elver <elver@xxxxxxxxxx>
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Tested-by: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
Signed-off-by: Jessica Yu <jeyu@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/module.c | 21 +++++++++++++++++++--
1 file changed, 19 insertions(+), 2 deletions(-)
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2107,6 +2107,21 @@ static int verify_export_symbols(struct
return 0;
}
+static bool ignore_undef_symbol(Elf_Half emachine, const char *name)
+{
+ /*
+ * On x86, PIC code and Clang non-PIC code may have call foo@PLT. GNU as
+ * before 2.37 produces an unreferenced _GLOBAL_OFFSET_TABLE_ on x86-64.
+ * i386 has a similar problem but may not deserve a fix.
+ *
+ * If we ever have to ignore many symbols, consider refactoring the code to
+ * only warn if referenced by a relocation.
+ */
+ if (emachine == EM_386 || emachine == EM_X86_64)
+ return !strcmp(name, "_GLOBAL_OFFSET_TABLE_");
+ return false;
+}
+
/* Change all symbols so that st_value encodes the pointer directly. */
static int simplify_symbols(struct module *mod, const struct load_info *info)
{
@@ -2148,8 +2163,10 @@ static int simplify_symbols(struct modul
break;
}
- /* Ok if weak. */
- if (!ksym && ELF_ST_BIND(sym[i].st_info) == STB_WEAK)
+ /* Ok if weak or ignored. */
+ if (!ksym &&
+ (ELF_ST_BIND(sym[i].st_info) == STB_WEAK ||
+ ignore_undef_symbol(info->hdr->e_machine, name)))
break;
pr_warn("%s: Unknown symbol %s (err %li)\n",
Patches currently in stable-queue which might be from maskray@xxxxxxxxxx are
queue-4.4/module-ignore-_global_offset_table_-when-warning-for-undefined-symbols.patch
