From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by sourceware.org (Postfix) with ESMTPS id CE5D0384BC21 for ; Fri, 28 Aug 2020 14:31:00 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org CE5D0384BC21 Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=suse.de Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=tdevries@suse.de X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id B3BF4B721; Fri, 28 Aug 2020 14:31:32 +0000 (UTC) Subject: Re: [PATCH][gdb/breakpoint] Handle setting breakpoint on label without address From: Tom de Vries To: Pedro Alves , gdb-patches@sourceware.org References: <20200827115217.GA17450@delia.home> <23b43acc-9846-a03c-5207-3ae80efc1d6f@palves.net> <29930a75-5de9-43ed-6a24-2909eb70ec66@suse.de> <7efc1cdf-5e48-9d46-0182-6bc2318d914b@palves.net> Autocrypt: addr=tdevries@suse.de; keydata= xsBNBF0ltCcBCADDhsUnMMdEXiHFfqJdXeRvgqSEUxLCy/pHek88ALuFnPTICTwkf4g7uSR7 HvOFUoUyu8oP5mNb4VZHy3Xy8KRZGaQuaOHNhZAT1xaVo6kxjswUi3vYgGJhFMiLuIHdApoc u5f7UbV+egYVxmkvVLSqsVD4pUgHeSoAcIlm3blZ1sDKviJCwaHxDQkVmSsGXImaAU+ViJ5l CwkvyiiIifWD2SoOuFexZyZ7RUddLosgsO0npVUYbl6dEMq2a5ijGF6/rBs1m3nAoIgpXk6P TCKlSWVW6OCneTaKM5C387972qREtiArTakRQIpvDJuiR2soGfdeJ6igGA1FZjU+IsM5ABEB AAHNH1RvbSBkZSBWcmllcyA8dGRldnJpZXNAc3VzZS5kZT7CwKsEEwEIAD4WIQSsnSe5hKbL MK1mGmjuhV2rbOJEoAUCXSW0JwIbAwUJA8JnAAULCQgHAgYVCgkICwIEFgIDAQIeAQIXgAAh CRDuhV2rbOJEoBYhBKydJ7mEpsswrWYaaO6FXats4kSgc48H/Ra2lq5p3dHsrlQLqM7N68Fo eRDf3PMevXyMlrCYDGLVncQwMw3O/AkousktXKQ42DPJh65zoXB22yUt8m0g12xkLax98KFJ 5NyUloa6HflLl+wQL/uZjIdNUQaHQLw3HKwRMVi4l0/Jh/TygYG1Dtm8I4o708JS4y8GQxoQ UL0z1OM9hyM3gI2WVTTyprsBHy2EjMOu/2Xpod95pF8f90zBLajy6qXEnxlcsqreMaqmkzKn 3KTZpWRxNAS/IH3FbGQ+3RpWkNGSJpwfEMVCeyK5a1n7yt1podd1ajY5mA1jcaUmGppqx827 8TqyteNe1B/pbiUt2L/WhnTgW1NC1QDOwE0EXSW0JwEIAM99H34Bu4MKM7HDJVt864MXbx7B 1M93wVlpJ7Uq+XDFD0A0hIal028j+h6jA6bhzWto4RUfDl/9mn1StngNVFovvwtfzbamp6+W pKHZm9X5YvlIwCx131kTxCNDcF+/adRW4n8CU3pZWYmNVqhMUiPLxElA6QhXTtVBh1RkjCZQ Kmbd1szvcOfaD8s+tJABJzNZsmO2hVuFwkDrRN8Jgrh92a+yHQPd9+RybW2l7sJv26nkUH5Z 5s84P6894ebgimcprJdAkjJTgprl1nhgvptU5M9Uv85Pferoh2groQEAtRPlCGrZ2/2qVNe9 XJfSYbiyedvApWcJs5DOByTaKkcAEQEAAcLAkwQYAQgAJhYhBKydJ7mEpsswrWYaaO6FXats 4kSgBQJdJbQnAhsMBQkDwmcAACEJEO6FXats4kSgFiEErJ0nuYSmyzCtZhpo7oVdq2ziRKD3 twf7BAQBZ8TqR812zKAD7biOnWIJ0McV72PFBxmLIHp24UVe0ZogtYMxSWKLg3csh0yLVwc7 H3vldzJ9AoK3Qxp0Q6K/rDOeUy3HMqewQGcqrsRRh0NXDIQk5CgSrZslPe47qIbe3O7ik/MC q31FNIAQJPmKXX25B115MMzkSKlv4udfx7KdyxHrTSkwWZArLQiEZj5KG4cCKhIoMygPTA3U yGaIvI/BGOtHZ7bEBVUCFDFfOWJ26IOCoPnSVUvKPEOH9dv+sNy7jyBsP5QxeTqwxC/1ZtNS DUCSFQjqA6bEGwM22dP8OUY6SC94x1G81A9/xbtm9LQxKm0EiDH8KBMLfQ== Message-ID: Date: Fri, 28 Aug 2020 16:30:58 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-6.3 required=5.0 tests=BAYES_00, KAM_DMARC_STATUS, NICE_REPLY_A, RCVD_IN_MSPIKE_H3, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: gdb-patches@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gdb-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 28 Aug 2020 14:31:02 -0000 On 8/28/20 3:53 PM, Tom de Vries wrote: > On 8/28/20 3:32 PM, Pedro Alves wrote: >> On 8/27/20 2:49 PM, Tom de Vries wrote: >>> On 8/27/20 2:41 PM, Pedro Alves wrote: >>>> On 8/27/20 12:52 PM, Tom de Vries wrote: >>>>> Hi, >>>>> >>>>> Consider test-case test.c: >>>>> ... >>>>> $ cat test.c >>>>> int main (void) { >>>>> return 0; >>>>> L1: >>>>> (void)0; >>>>> } >>>>> ... >>>>> >>>>> Compiled with debug info: >>>>> ... >>>>> $ gcc test.c -g >>>>> ... >>>>> >>>>> When attempting to set a breakpoint at L1, which is a label without address: >>>>> ... >>>>> <1>: Abbrev Number: 2 (DW_TAG_subprogram) >>>>> DW_AT_name : main >>>>> <2><115>: Abbrev Number: 3 (DW_TAG_label) >>>>> <116> DW_AT_name : L1 >>>>> <119> DW_AT_decl_file : 1 >>>>> <11a> DW_AT_decl_line : 5 >>>>> <2><11b>: Abbrev Number: 0 >>>> >>>> Is this a debug info bug, >>> >>> Strictly speaking, this is a debug info bug. The standard says that: >>> ... >>> The label entry has a DW_AT_low_pc attribute whose value is the address >>> of the first executable instruction for the location identified by the >>> label in the source program. >>> ... >>> >>> But I interpret the missing DW_AT_low_pc attribute as: there is a label >>> in the source, but the corresponding code has been optimized out. >>> >>>> or is the debug info telling us that the >>>> address of the label is the same as the line number's address? >>>> >>>> How about looking up the line number address instead of throwing >>>> an error? >>>> >>> >>> Well, in this particular case, that wouldn't help. >>> >>> With L1 at line 3: >>> ... >>> $ cat -n test.c >>> 1 int main (void) { >>> 2 return 0; >>> 3 L1: >>> 4 (void)0; >>> 5 } >>> 6 >>> ... >>> there's no corresponding address: >>> ... >>> $ readelf -wL a.out >>> CU: test.c: >>> File name Line number Starting address >>> View Stmt >>> test.c 1 0x400497 >>> x >>> test.c 2 0x40049b >>> x >>> test.c 5 0x4004a0 >>> x >>> test.c - 0x4004a2 >>> ... >>> >>> My suspicion is that this won't be useful in general. >> >> I don't understand the "not useful" remark. If a user does gets >> the error, they'll probably do: >> >> "b 3", >> >> and they'll get a breakpoint at line 5, no? >> >> That's very likely what a user would do after the error. >> >> IMO GDB should do that for the user. >> >> So far I don't agree with your patch. >> > > I see what you mean, but let's try this counter-example: > ... > cat -n test.c > 1 int > 2 main (void) > 3 { > 4 goto L2; > 5 > 6 L3: > 7 return 0; > 8 > 9 L1: > 10 (void)0; > 11 return 1; > 12 > 13 L2: > 14 goto L3; > 15 } > 16 > ... > compiled like this: > ... > $ gcc test.c -g > ... > > With the patch, we're not able to set a breakpoint at L1, and setting > the breakpoint at the corresponding line, line 9: > ... > $ gdb a.out > Reading symbols from a.out... > (gdb) b main:L1 > Location main:L1 not available > (gdb) b 9 > Breakpoint 1 at 0x40049c: file test.c, line 14. > (gdb) > ... > yields a breakpoint at line 14, a piece of code that's not reachable > from L1. > > To me, label L1 and line 14 are unrelated enough to convince me to not > do this automatically. > FWIW, lldb does the same: ... $ lldb a.out (lldb) target create "a.out" Current executable set to 'a.out' (x86_64). (lldb) b main:L1 Breakpoint 1: no locations (pending). WARNING: Unable to resolve breakpoint to any actual locations. (lldb) b 9 Breakpoint 2: where = a.out`main + 5 at test.c:14, address = 0x000000000040049c (lldb) ... Thanks, - Tom