From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001 From: Tom de Vries Date: Wed, 20 Jan 2021 16:29:30 +0100 Subject: [PATCH] [gdb/server] Don't overwrite fs/gs_base with -m32 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 * linux-x86-low.cc (collect_register_i386): New function. (x86_fill_gregset): Remove memset. Use collect_register_i386. --- gdbserver/ChangeLog | 5 ++++ gdbserver/linux-x86-low.cc | 55 +++++++++++++++++++++++--------------- 2 files changed, 38 insertions(+), 22 deletions(-) diff --git a/gdbserver/ChangeLog b/gdbserver/ChangeLog index 28d0f9bfccb..b87174ce4e3 100644 --- a/gdbserver/ChangeLog +++ b/gdbserver/ChangeLog @@ -1,3 +1,8 @@ +2021-01-20 Tom de Vries + + * linux-x86-low.cc (collect_register_i386): New function. + (x86_fill_gregset): Remove memset. Use collect_register_i386. + 2021-01-01 Joel Brobecker * server.cc (gdbserver_version): Update copyright year. diff --git a/gdbserver/linux-x86-low.cc b/gdbserver/linux-x86-low.cc index 79284473f14..ea95f3ea2ae 100644 --- 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 -- 2.29.2