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Levin" , Christophe Leroy Cc: Alexey Gladkov , Oleg Nesterov , Michael Ellerman , Eugene Syromyatnikov , Mike Frysinger , Renzo Davoli , Davide Berardi , strace-devel@lists.strace.io, Nicholas Piggin , Naveen N Rao , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org References: <20250113171054.GA589@strace.io> <6558110c-c2cb-4aa3-9472-b3496f71ebb8@csgroup.eu> <20250114170400.GB11820@strace.io> <20250120171249.GA17320@strace.io> Content-Language: en-US From: Madhavan Srinivasan In-Reply-To: <20250120171249.GA17320@strace.io> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: GCBA0hxG_lJipEZlbfLWZUfw0KnA0eQV X-Proofpoint-ORIG-GUID: tSyCLZalsMRyNo_lRGv_2BXPVhPchd6f X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1057,Hydra:6.0.680,FMLib:17.12.68.34 definitions=2025-01-21_05,2025-01-21_02,2024-11-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 priorityscore=1501 suspectscore=0 mlxlogscore=999 bulkscore=0 phishscore=0 clxscore=1011 impostorscore=0 mlxscore=0 adultscore=0 spamscore=0 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2411120000 definitions=main-2501210091 On 1/20/25 10:42 PM, Dmitry V. Levin wrote: > On Mon, Jan 20, 2025 at 02:51:38PM +0100, Christophe Leroy wrote: >> Le 14/01/2025 à 18:04, Dmitry V. Levin a écrit : >>> On Mon, Jan 13, 2025 at 06:34:44PM +0100, Christophe Leroy wrote: >>>> Le 13/01/2025 à 18:10, Dmitry V. Levin a écrit : >>>>> Bring syscall_set_return_value() in sync with syscall_get_error(), >>>>> and let upcoming ptrace/set_syscall_info selftest pass on powerpc. >>>>> Sorry for getting to this thread late. Tried the series without this patch in 1) power9 PowerNV system and in power10 pSeries lpar # ./set_syscall_info TAP version 13 1..1 # Starting 1 tests from 1 test cases. # RUN global.set_syscall_info ... # OK global.set_syscall_info ok 1 global.set_syscall_info # PASSED: 1 / 1 tests passed. # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0 and in both case set_syscall_info passes. Will look at it further. Maddy >>>>> This reverts commit 1b1a3702a65c ("powerpc: Don't negate error in >>>>> syscall_set_return_value()"). >>>> >>>> There is a clear detailed explanation in that commit of why it needs to >>>> be done. >>>> >>>> If you think that commit is wrong you have to explain why with at least >>>> the same level of details. >>> >>> OK, please have a look whether this explanation is clear and detailed enough: >>> >>> ======= >>> powerpc: properly negate error in syscall_set_return_value() >>> >>> When syscall_set_return_value() is used to set an error code, the caller >>> specifies it as a negative value in -ERRORCODE form. >>> >>> In !trap_is_scv case the error code is traditionally stored as follows: >>> gpr[3] contains a positive ERRORCODE, and ccr has 0x10000000 flag set. >>> Here are a few examples to illustrate this convention. The first one >>> is from syscall_get_error(): >>> /* >>> * If the system call failed, >>> * regs->gpr[3] contains a positive ERRORCODE. >>> */ >>> return (regs->ccr & 0x10000000UL) ? -regs->gpr[3] : 0; >>> >>> The second example is from regs_return_value(): >>> if (is_syscall_success(regs)) >>> return regs->gpr[3]; >>> else >>> return -regs->gpr[3]; >>> >>> The third example is from check_syscall_restart(): >>> regs->result = -EINTR; >>> regs->gpr[3] = EINTR; >>> regs->ccr |= 0x10000000; >>> >>> Compared with these examples, the failure of syscall_set_return_value() >>> to assign a positive ERRORCODE into regs->gpr[3] is clearly visible: >>> /* >>> * In the general case it's not obvious that we must deal with >>> * CCR here, as the syscall exit path will also do that for us. >>> * However there are some places, eg. the signal code, which >>> * check ccr to decide if the value in r3 is actually an error. >>> */ >>> if (error) { >>> regs->ccr |= 0x10000000L; >>> regs->gpr[3] = error; >>> } else { >>> regs->ccr &= ~0x10000000L; >>> regs->gpr[3] = val; >>> } >>> >>> This fix brings syscall_set_return_value() in sync with syscall_get_error() >>> and lets upcoming ptrace/set_syscall_info selftest pass on powerpc. >>> >>> Fixes: 1b1a3702a65c ("powerpc: Don't negate error in syscall_set_return_value()"). >>> ======= >>> >>> >> >> I think there is still something going wrong. >> >> do_seccomp() sets regs->gpr[3] = -ENOSYS; by default. >> >> Then it calls __secure_computing() which returns what __seccomp_filter() >> returns. >> >> In case of error, __seccomp_filter() calls syscall_set_return_value() >> with a negative value then returns -1 >> >> do_seccomp() is called by do_syscall_trace_enter() which returns -1 when >> do_seccomp() doesn't return 0. >> >> do_syscall_trace_enter() is called by system_call_exception() and >> returns -1, so syscall_exception() returns regs->gpr[3] >> >> In entry_32.S, transfer_to_syscall, syscall_exit_prepare() is then >> called with the return of syscall_exception() as first parameter, which >> leads to: >> >> if (unlikely(r3 >= (unsigned long)-MAX_ERRNO) && is_not_scv) { >> if (likely(!(ti_flags & (_TIF_NOERROR | _TIF_RESTOREALL)))) { >> r3 = -r3; >> regs->ccr |= 0x10000000; /* Set SO bit in CR */ >> } >> } > > Note the "unlikely" keyword here reminding us once more that in !scv case > regs->gpr[3] does not normally have -ERRORCODE form. > >> By chance, because you have already changed the sign of gpr[3], the >> above test fails and nothing is done to r3, and because you have also >> already set regs->ccr it works. >> >> But all this looks inconsistent with the fact that do_seccomp sets >> -ENOSYS as default value >> >> Also, when do_seccomp() returns 0, do_syscall_trace_enter() check the >> syscall number and when it is wrong it goes to skip: which sets >> regs->gpr[3] = -ENOSYS; > > It looks like do_seccomp() and do_syscall_trace_enter() get away by sheer > luck, implicitly relying on syscall_exit_prepare() transparently fixing > regs->gpr[3] for them. > >> So really I think it is not in line with your changes to set positive >> value in gpr[3]. >> >> Maybe your change is still correct but it needs to be handled completely >> in that case. > > By the way, is there any reasons why do_seccomp() and > do_syscall_trace_enter() don't use syscall_set_return_value() yet? > >