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[109.81.82.22]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-440a530a6e9sm26807935e9.16.2025.04.25.07.37.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 25 Apr 2025 07:37:26 -0700 (PDT) Date: Fri, 25 Apr 2025 16:37:47 +0200 From: Jan Stancek To: Thomas =?iso-8859-1?Q?Wei=DFschuh?= Cc: Andy Lutomirski , Thomas Gleixner , Vincenzo Frascino , Catalin Marinas , Will Deacon , Anna-Maria Behnsen , Frederic Weisbecker , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Naveen N Rao , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Arnd Bergmann , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-arch@vger.kernel.org, Nam Cao Subject: Re: [PATCH 08/19] vdso/gettimeofday: Prepare do_hres_timens() for introduction of struct vdso_clock Message-ID: References: <20250303-vdso-clock-v1-0-c1b5c69a166f@linutronix.de> <20250303-vdso-clock-v1-8-c1b5c69a166f@linutronix.de> <20250424173908-ffca1ea2-e292-4df3-9391-24bfdaab33e7@linutronix.de> <20250425104552-07539a73-8f56-44d2-97a2-e224c567a2fc@linutronix.de> <20250425152733-0ff10421-b716-4a55-9b60-cb0a71769e56@linutronix.de> Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20250425152733-0ff10421-b716-4a55-9b60-cb0a71769e56@linutronix.de> On Fri, Apr 25, 2025 at 03:40:55PM +0200, Thomas Weißschuh wrote: > >Some more information: > >The crash comes from the address arithmetic in "vc = &vc[CS_RAW]" going wrong. That appears to be because it's not doing any arithmetic, but using value from some linker-generated symbol (I'll refer to it as "7a8"). The below is presumably the check that compares clk != CLOCK_MONOTONIC_RAW, CLOCK_MONOTONIC_RAW is 4. And it should choose between first and second vdso_clock in 2nd vvar page: # readelf -a /root/kernel-ark/arch/arm64/kernel/vdso/vdso.so.dbg | grep 7a8 25: 00000000000007a8 0 NOTYPE LOCAL DEFAULT 7 $d objdump -d -S: 658: 17ffffef b 614 <__cvdso_clock_gettime_data.constprop.0+0x104> vc = &vc[CS_HRES_COARSE]; 65c: 58000a62 ldr x2, 7a8 <__cvdso_clock_gettime_data.constprop.0+0x298> 660: 7100101f cmp w0, #0x4 664: 10e7cce3 adr x3, fffffffffffd0000 668: 9a821063 csel x3, x3, x2, ne // ne = any while (unlikely((seq = READ_ONCE(vc->seq)) & 1)) 66c: b9400065 ldr w5, [x3] ... 7a8: fffd00e0 .word 0xfffd00e0 7ac: ffffffff .word 0xffffffff (gdb) r Thread 2.1 "a.out" received signal SIGSEGV, Segmentation fault. [Switching to Thread 0xfffff7ff2dc0 (LWP 44638)] 0x0000fffff7fa066c in ?? () (gdb) disassemble 0x0000fffff7fa0658,0x0000fffff7fa067c --Type for more, q to quit, c to continue without paging-- 0x0000fffff7fa0658: b 0xfffff7fa0614 0x0000fffff7fa065c: ldr x2, 0xfffff7fa07a8 0x0000fffff7fa0660: cmp w0, #0x4 0x0000fffff7fa0664: adr x3, 0xfffff7f70000 0x0000fffff7fa0668: csel x3, x3, x2, ne // ne = any => 0x0000fffff7fa066c: ldr w5, [x3] $x3 here is using correct vvar runtime address, but $x2 uses a bogus one from "7a8". And when it tries to load from it, it crashes: (gdb) info proc map process 44638 Mapped address spaces: Start Addr End Addr Size Offset Perms File 0x0000000000400000 0x0000000000410000 0x10000 0x0 r-xp /root/ltp/testcases/kernel/syscalls/clock_gettime/a.out 0x0000000000410000 0x0000000000420000 0x10000 0x0 r--p /root/ltp/testcases/kernel/syscalls/clock_gettime/a.out 0x0000000000420000 0x0000000000430000 0x10000 0x10000 rw-p /root/ltp/testcases/kernel/syscalls/clock_gettime/a.out 0x0000fffff7d80000 0x0000fffff7f30000 0x1b0000 0x0 r-xp /usr/lib64/libc.so.6 0x0000fffff7f30000 0x0000fffff7f40000 0x10000 0x1a0000 r--p /usr/lib64/libc.so.6 0x0000fffff7f40000 0x0000fffff7f50000 0x10000 0x1b0000 rw-p /usr/lib64/libc.so.6 0x0000fffff7f60000 0x0000fffff7fa0000 0x40000 0x0 r--p [vvar] 0x0000fffff7fa0000 0x0000fffff7fb0000 0x10000 0x0 r-xp [vdso] 0x0000fffff7fb0000 0x0000fffff7fe0000 0x30000 0x0 r-xp /usr/lib/ld-linux-aarch64.so.1 0x0000fffff7fe0000 0x0000fffff7ff0000 0x10000 0x20000 r--p /usr/lib/ld-linux-aarch64.so.1 0x0000fffff7ff0000 0x0000fffff8000000 0x10000 0x30000 rw-p /usr/lib/ld-linux-aarch64.so.1 0x0000fffffffd0000 0x0001000000000000 0x30000 0x0 rw-p [stack] $x3 was the beginning of 2nd vvar page (1st vdso_clock), and $x2 looks like it should have been 2nd vdso_clock, but the value that is used is not the address of vvar during runtime. vdso_clock has size 224 == 0xe0 (according to pahole) (gdb) x/2x 0xfffff7fa07a8 0xfffff7fa07a8: 0xfffd00e0 0xffffffff (gdb) p/x $x2 $1 = 0xfffffffffffd00e0 (gdb) x/1x $x2 0xfffffffffffd00e0: Cannot access memory at address 0xfffffffffffd00e0 But it does match the symbol value from vdso.so: # readelf -a /root/kernel-ark/arch/arm64/kernel/vdso/vdso.so.dbg | grep vdso_u 37: fffffffffffc0000 0 NOTYPE LOCAL DEFAULT 1 vdso_u_data 39: fffffffffffc0000 0 NOTYPE LOCAL DEFAULT ABS vdso_u_time_data 40: fffffffffffe0000 0 NOTYPE LOCAL DEFAULT ABS vdso_u_rng_data >>> print(hex(0xfffffffffffc0000+65536+224)) 0xfffffffffffd00e0 -> and this is address where it crashes Regards, Jan >This should just do "vc = vc + 1", advancing the pointer by sizeof(*vc). >But I get these example values of "vc" before and after the arithmetic: >0x00ffffbc060000 -> 0xfffffffffffd00e8 >Which is obviously wrong. > >The arithmetic can be fixed with any one of the following changes: >* OPTIMIZER_HIDE_VAR() as above >* Replacement of -mcmodel=tiny with -mcmode=small in arch/arm64/kernel/vdso/Makefile > -mcmodel=tiny is supposed to cover 1MiB programs, the vDSO only needs 300KiB here >* Removal of the (clk != CLOCK_MONOTONIC_RAW) case > >Shuffling code around, even without any impact on semantics, sometimes fixes >the issue. Compiling the vDSO with UBSAN didn't show anything. > > >Thomas > >> > > > > [1] https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/syscalls/clock_gettime/clock_gettime03.c >> > > > > [2] https://koji.fedoraproject.org/koji/buildinfo?buildID=2704401 >> > > > >> > >> >