From patchwork Sun Mar 17 05:13:41 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Kevin Buettner X-Patchwork-Id: 31872 Received: (qmail 6867 invoked by alias); 17 Mar 2019 05:13:47 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Delivered-To: mailing list gdb-patches@sourceware.org Received: (qmail 6837 invoked by uid 89); 17 Mar 2019 05:13:45 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-15.9 required=5.0 tests=AWL, BAYES_00, GIT_PATCH_0, GIT_PATCH_1, GIT_PATCH_2, GIT_PATCH_3, SPF_HELO_PASS autolearn=ham version=3.3.1 spammy=fill, carrying, seemed, store_registers X-HELO: mx1.redhat.com Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Sun, 17 Mar 2019 05:13:43 +0000 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 1E212307D93E for ; Sun, 17 Mar 2019 05:13:42 +0000 (UTC) Received: from f29-4.lan (ovpn-117-11.phx2.redhat.com [10.3.117.11]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EF04B1001DE1 for ; Sun, 17 Mar 2019 05:13:41 +0000 (UTC) Date: Sat, 16 Mar 2019 22:13:41 -0700 From: Kevin Buettner To: gdb-patches@sourceware.org Subject: [PATCH] Fix amd64->i386 linux syscall restart problem Message-ID: <20190316221341.021f7c62@f29-4.lan> MIME-Version: 1.0 X-IsSubscribed: yes This commit fixes some failures in gdb.base/interrupt.exp when debugging a 32-bit i386 linux inferior from an amd64 host. When running the following test... make check RUNTESTFLAGS="--target_board unix/-m32 interrupt.exp" ... without this commit, I see the following output: FAIL: gdb.base/interrupt.exp: continue (the program exited) FAIL: gdb.base/interrupt.exp: echo data FAIL: gdb.base/interrupt.exp: Send Control-C, second time FAIL: gdb.base/interrupt.exp: signal SIGINT (the program is no longer running) ERROR: Undefined command "". ERROR: GDB process no longer exists === gdb Summary === When the test is run with this commit in place, we see 12 passes instead. This is the desired behavior. Analysis: On Linux, when a syscall is interrupted by a signal, the syscall may return -ERESTARTSYS when a signal occurs. Doing so indicates that the syscall is restartable. Then, depending on settings associated with the signal handler, and after the signal handler is called, the kernel can then either return -EINTR or can cause the syscall to be restarted. In this discussion, we are concerned with the latter case. On i386, the kernel returns this status via the EAX register. When debugging a 32-bit (i386) process from a 64-bit (amd64) GDB, the debugger fetches 64-bit registers even though the process being debugged is 32-bit. Since we're debugging a 32-bit target, only 32 bits are being saved in the register cache. Now, ideally, GDB would save all 64-bits in the regcache and then would be able to restore those same values when it comes time to continue the target. I've looked into doing this, but it's not easy and I don't see many benefits to doing so. One benefit, however, would be that EAX would appear as a negative value for doing syscall restarts. At the moment, GDB is setting the high 32 bits of RAX (and other registers too) to 0. So, when GDB restores EAX just prior to a syscall restart, the high 32 bits of RAX are zeroed, thus making it look like a positive value. For this particular purpose, we need to sign extend EAX so that RAX will appear as a negative value when EAX is set to -ERESTARTSYS. This in turn will cause the signal handling code in the kernel to recognize -ERESTARTSYS which will in turn cause the syscall to be restarted. This commit is based on work by Jan Kratochvil from 2009: https://sourceware.org/ml/gdb-patches/2009-11/msg00592.html Jan's patch had the sign extension code in amd64-nat.c. Several other native targets make use of this code, so it seemed better to move the sign extension code to a linux specific file. I also added similar code to gdbserver. Another approach is to fix the problem in the kernel. Hui Zhu tried to get a fix into the kernel back in 2014, but it was not accepted. Discussion regarding this approach may be found here: https://lore.kernel.org/patchwork/patch/457841/ Even if a fix were to be put into the kernel, we'd still need some kind of fix in GDB in order to support older kernels. Finally, I'll note that Fedora has been carrying a similar patch for at least nine years. Other distributions, including RHEL and CentOS have picked up this change and have been using it too. gdb/ChangeLog: * amd64-linux-nat.c (amd64_linux_collect_native_gregset): New function. (fill_gregset): Call amd64_linux_collect_native_gregset instead of amd64_collect_native_gregset. (amd64_linux_nat_target::store_registers): Likewise. gdb/gdbserver/ChangeLog: * linux-x86-low.c (x86_fill_gregset): Sign extend EAX value when using a 64-bit gdbserver. --- gdb/amd64-linux-nat.c | 32 ++++++++++++++++++++++++++++++-- gdb/gdbserver/linux-x86-low.c | 9 +++++++++ 2 files changed, 39 insertions(+), 2 deletions(-) diff --git a/gdb/amd64-linux-nat.c b/gdb/amd64-linux-nat.c index a0bb105f5a..06018c8f1c 100644 --- a/gdb/amd64-linux-nat.c +++ b/gdb/amd64-linux-nat.c @@ -92,6 +92,34 @@ static int amd64_linux_gregset32_reg_offset[] = /* Transfering the general-purpose registers between GDB, inferiors and core files. */ +/* See amd64_collect_native_gregset. This linux specific version handles + issues with negative EAX values not being restored correctly upon syscall + return when debugging 32-bit targets. It has no effect on 64-bit + targets. */ + +static void +amd64_linux_collect_native_gregset (const struct regcache *regcache, + void *gregs, int regnum) +{ + amd64_collect_native_gregset (regcache, gregs, regnum); + + struct gdbarch *gdbarch = regcache->arch (); + if (gdbarch_bfd_arch_info (gdbarch)->bits_per_word == 32) + { + /* Sign-extend %eax as during return from a syscall it is being checked + for -ERESTART* values -512 being above 0xfffffffffffffe00; tested by + interrupt.exp. */ + + if (regnum == -1 || regnum == I386_EAX_REGNUM) + { + void *ptr = (gdb_byte *) gregs + + amd64_linux_gregset32_reg_offset[I386_EAX_REGNUM]; + + *(int64_t *) ptr = *(int32_t *) ptr; + } + } +} + /* Fill GDB's register cache with the general-purpose register values in *GREGSETP. */ @@ -109,7 +137,7 @@ void fill_gregset (const struct regcache *regcache, elf_gregset_t *gregsetp, int regnum) { - amd64_collect_native_gregset (regcache, gregsetp, regnum); + amd64_linux_collect_native_gregset (regcache, gregsetp, regnum); } /* Transfering floating-point registers between GDB, inferiors and cores. */ @@ -237,7 +265,7 @@ amd64_linux_nat_target::store_registers (struct regcache *regcache, int regnum) if (ptrace (PTRACE_GETREGS, tid, 0, (long) ®s) < 0) perror_with_name (_("Couldn't get registers")); - amd64_collect_native_gregset (regcache, ®s, regnum); + amd64_linux_collect_native_gregset (regcache, ®s, regnum); if (ptrace (PTRACE_SETREGS, tid, 0, (long) ®s) < 0) perror_with_name (_("Couldn't write registers")); diff --git a/gdb/gdbserver/linux-x86-low.c b/gdb/gdbserver/linux-x86-low.c index 029796e361..20f369c496 100644 --- a/gdb/gdbserver/linux-x86-low.c +++ b/gdb/gdbserver/linux-x86-low.c @@ -338,6 +338,15 @@ x86_fill_gregset (struct regcache *regcache, void *buf) collect_register_by_name (regcache, "orig_eax", ((char *) buf) + ORIG_EAX * REGSIZE); + + /* Sign extend EAX value to avoid potential syscall restart problems. */ + 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; + } } static void