gdb/SOURCES/gdb-dont-overwrite-fsgsbase-m32.patch

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2021-11-03 14:59:45 +00:00
From FEDORA_PATCHES Mon Sep 17 00:00:00 2001
From: Tom de Vries <tdevries@suse.de>
Date: Tue, 1 Jun 2021 10:14:31 -0700
Subject: gdb-dont-overwrite-fsgsbase-m32.patch
;; Backport "[gdb/server] Don't overwrite fs/gs_base with -m32"
;; (Tom de Vries)
Consider a minimal test-case test.c:
...
int main (void) { return 0; }
...
compiled with -m32:
...
$ gcc test.c -m32
...
When running the exec using gdbserver on openSUSE Factory (currently running a
linux kernel version 5.10.5):
...
$ gdbserver localhost:12345 a.out
...
to which we connect in a gdb session, we run into a segfault in the inferior:
...
$ gdb -batch -q -ex "target remote localhost:12345" -ex continue
Program received signal SIGSEGV, Segmentation fault.
0xf7dd8bd2 in init_cacheinfo () at ../sysdeps/x86/cacheinfo.c:761
...
The segfault is caused by gdbserver overwriting $gs_base with 0 using
PTRACE_SETREGS. After it is overwritten, the next use of $gs in the inferior
will trigger the segfault.
Before linux kernel version 5.9, the value used by PTRACE_SETREGS for $gs_base
was ignored, but starting version 5.9, the linux kernel has support for
intel architecture extension FSGSBASE, which allows users to modify $gs_base,
and consequently PTRACE_SETREGS can no longer ignore the $gs_base value.
The overwrite of $gs_base with 0 is done by a memset in x86_fill_gregset,
which was added in commit 9e0aa64f551 "Fix gdbserver qGetTLSAddr for
x86_64 -m32". The memset intends to zero-extend 32-bit registers that are
tracked in the regcache to 64-bit when writing them into the PTRACE_SETREGS
data argument. But in addition, it overwrites other registers that are
not tracked in the regcache, such as $gs_base.
Fix the segfault by redoing the fix from commit 9e0aa64f551 in minimal form.
Tested on x86_64-linux:
- openSUSE Leap 15.2 (using kernel version 5.3.18):
- native
- gdbserver -m32
- -m32
- openSUSE Factory (using kernel version 5.10.5):
- native
- m32
gdbserver/ChangeLog:
2021-01-20 Tom de Vries <tdevries@suse.de>
* linux-x86-low.cc (collect_register_i386): New function.
(x86_fill_gregset): Remove memset. Use collect_register_i386.
diff --git a/gdbserver/linux-x86-low.cc b/gdbserver/linux-x86-low.cc
--- a/gdbserver/linux-x86-low.cc
+++ b/gdbserver/linux-x86-low.cc
@@ -397,6 +397,35 @@ x86_target::low_cannot_fetch_register (int regno)
return regno >= I386_NUM_REGS;
}
+static void
+collect_register_i386 (struct regcache *regcache, int regno, void *buf)
+{
+ collect_register (regcache, regno, buf);
+
+#ifdef __x86_64__
+ /* In case of x86_64 -m32, collect_register only writes 4 bytes, but the
+ space reserved in buf for the register is 8 bytes. Make sure the entire
+ reserved space is initialized. */
+
+ gdb_assert (register_size (regcache->tdesc, regno) == 4);
+
+ if (regno == RAX)
+ {
+ /* Sign extend EAX value to avoid potential syscall restart
+ problems.
+
+ See amd64_linux_collect_native_gregset() in
+ gdb/amd64-linux-nat.c for a detailed explanation. */
+ *(int64_t *) buf = *(int32_t *) buf;
+ }
+ else
+ {
+ /* Zero-extend. */
+ *(uint64_t *) buf = *(uint32_t *) buf;
+ }
+#endif
+}
+
static void
x86_fill_gregset (struct regcache *regcache, void *buf)
{
@@ -411,32 +440,14 @@ x86_fill_gregset (struct regcache *regcache, void *buf)
return;
}
-
- /* 32-bit inferior registers need to be zero-extended.
- Callers would read uninitialized memory otherwise. */
- memset (buf, 0x00, X86_64_USER_REGS * 8);
#endif
for (i = 0; i < I386_NUM_REGS; i++)
- collect_register (regcache, i, ((char *) buf) + i386_regmap[i]);
-
- collect_register_by_name (regcache, "orig_eax",
- ((char *) buf) + ORIG_EAX * REGSIZE);
+ collect_register_i386 (regcache, i, ((char *) buf) + i386_regmap[i]);
-#ifdef __x86_64__
- /* Sign extend EAX value to avoid potential syscall restart
- problems.
-
- See amd64_linux_collect_native_gregset() in gdb/amd64-linux-nat.c
- for a detailed explanation. */
- if (register_size (regcache->tdesc, 0) == 4)
- {
- void *ptr = ((gdb_byte *) buf
- + i386_regmap[find_regno (regcache->tdesc, "eax")]);
-
- *(int64_t *) ptr = *(int32_t *) ptr;
- }
-#endif
+ /* Handle ORIG_EAX, which is not in i386_regmap. */
+ collect_register_i386 (regcache, find_regno (regcache->tdesc, "orig_eax"),
+ ((char *) buf) + ORIG_EAX * REGSIZE);
}
static void