[{"id":3680714,"web_url":"http://patchwork.ozlabs.org/comment/3680714/","msgid":"<874il3lyig.fsf@gmail.com>","list_archive_url":null,"date":"2026-04-22T17:01:59","subject":"Re: [PATCH v2] misc: Optimize getusershell.c","submitter":{"id":88687,"url":"http://patchwork.ozlabs.org/api/people/88687/","name":"Collin Funk","email":"collin.funk1@gmail.com"},"content":"Rocket Ma writes:\n\n> * misc/getusershell.c: Completely rewrite the unit. Only allocate one\n> big buffer to store shell names. Add a missing unit test.\n>\n> The new implementation read the whole file into one buffer, and wipe out\n> every byte but shell names. Later when addressing shell names from first\n> shell, jump to next '\\0' and then jump to next '/'. This could reduce\n> memory footprint and shall improve some performance.\n>\n> Signed-off-by: Rocket Ma \n> ---\n> This new patch fixed some unexpected plt links, and use isspace_l to\n> force parsing /etc/shells with C locale. (Is that necessary?)\n\nThanks for the patch.\n\nHowever, I am against this change. The /etc/shells file can be\narbitrarily long, so we shouldn't read it all into memory. It also\nchanges the behavior of the function to be different to all the BSD\nversions. In this version appending a shell while the process is running\nwill not affect subsequent calls to getusershell.\n\n> + if ((fp = fopen (_PATH_SHELLS, \"rce\")) == NULL)\n> + goto default_out;\n> + if ((__fstat64_time64 (__fileno (fp), &fstat)) == -1)\n> + goto close_out;\n> + /* Consider if buflen will overflow. */\n> + if (fstat.st_size < 2 || fstat.st_size > PTRDIFF_MAX - 1)\n> + goto close_out;\n> + /* 1 byte for \\n (will be overwritten as \\0). */\n> + buflen = fstat.st_size + 1;\n> + if ((shellbuf = malloc (buflen)) == NULL)\n> + goto close_out;\n> + shellbuf[buflen - 1] = '\\n';\n> + _IO_setbuf (fp, NULL);\n> + if ((_IO_fread (shellbuf, 1, fstat.st_size, fp)) != fstat.st_size)\n> + goto free_out;\n\nWhat if we call fstat while a user is in the middle of adding a shell?\nE.g., then it is possible we return a truncated shell because we didn't\nallocate enough memory?\n\nCollin","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","libc-alpha@sourceware.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","libc-alpha@sourceware.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=X7lIfIwV;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=sourceware.org\n (client-ip=38.145.34.32; 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Only allocate one\n>> big buffer to store shell names. Add a missing unit test.\n>>\n>> The new implementation read the whole file into one buffer, and wipe out\n>> every byte but shell names. Later when addressing shell names from first\n>> shell, jump to next '\\0' and then jump to next '/'. This could reduce\n>> memory footprint and shall improve some performance.\n>>\n>> Signed-off-by: Rocket Ma \n>> ---\n>> This new patch fixed some unexpected plt links, and use isspace_l to\n>> force parsing /etc/shells with C locale. (Is that necessary?)\n> \n> Thanks for the patch.\n> \n> However, I am against this change. The /etc/shells file can be\n> arbitrarily long, so we shouldn't read it all into memory. It also\n> changes the behavior of the function to be different to all the BSD\n> versions. In this version appending a shell while the process is running\n> will not affect subsequent calls to getusershell.\n\nThe current code already read the whole file before parsing it, this\nnew approach is no worse. The BSD code also seems to limit each\nshell to PATH_MAX, while this approach seems to work in-place and\nremoves the extra buffer allocation.\n\n> \n>> + if ((fp = fopen (_PATH_SHELLS, \"rce\")) == NULL)\n>> + goto default_out;\n>> + if ((__fstat64_time64 (__fileno (fp), &fstat)) == -1)\n>> + goto close_out;\n>> + /* Consider if buflen will overflow. */\n>> + if (fstat.st_size < 2 || fstat.st_size > PTRDIFF_MAX - 1)\n>> + goto close_out;\n>> + /* 1 byte for \\n (will be overwritten as \\0). */\n>> + buflen = fstat.st_size + 1;\n>> + if ((shellbuf = malloc (buflen)) == NULL)\n>> + goto close_out;\n>> + shellbuf[buflen - 1] = '\\n';\n>> + _IO_setbuf (fp, NULL);\n>> + if ((_IO_fread (shellbuf, 1, fstat.st_size, fp)) != fstat.st_size)\n>> + goto free_out;\n> \n> What if we call fstat while a user is in the middle of adding a shell?\n> E.g., then it is possible we return a truncated shell because we didn't\n> allocate enough memory?\nI do not think we have this guarantee for current code, and I am not sure\nif would be feasible without some extra support OS.","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","libc-alpha@sourceware.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","libc-alpha@sourceware.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=linaro.org header.i=@linaro.org header.a=rsa-sha256\n header.s=google header.b=EL27XdBt;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=sourceware.org\n (client-ip=2620:52:6:3111::32; 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charset=UTF-8","Content-Transfer-Encoding":"7bit","X-BeenThere":"libc-alpha@sourceware.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Libc-alpha mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"libc-alpha-bounces~incoming=patchwork.ozlabs.org@sourceware.org"}},{"id":3680724,"web_url":"http://patchwork.ozlabs.org/comment/3680724/","msgid":"","list_archive_url":null,"date":"2026-04-22T17:32:23","subject":"Re: [PATCH v2] misc: Optimize getusershell.c","submitter":{"id":92898,"url":"http://patchwork.ozlabs.org/api/people/92898/","name":"Rocket Ma","email":"marocketbd@gmail.com"},"content":"> However, I am against this change. The /etc/shells file can be\n> arbitrarily long, so we shouldn't read it all into memory. It also\n> changes the behavior of the function to be different to all the BSD\n> versions. In this version appending a shell while the process is running\n> will not affect subsequent calls to getusershell.\n>\n> What if we call fstat while a user is in the middle of adding a shell?\n> E.g., then it is possible we return a truncated shell because we didn't\n> allocate enough memory?\n>\n> Collin\n\nFirst, since the old implementation also allocates a buffer with file\nsize, it assumes that the file should be small. Not to mention the\ndynamically allocated array offhead. Second, I tested the behavior\nlocally, if another process is manipulating /etc/shells, fgets will\nnot wait until the process is completed. Once fgets read all of the\nfile, even though some process is still appending, fgets returns NULL.\nSo the old implementation should face the same problem, too. Also,\n/etc/shells should be stable enough that we could read it in one\nfread. The race condition is so rare that it could be treated as an\nundefined behavior.\n\nRocket","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","libc-alpha@sourceware.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","libc-alpha@sourceware.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=QBOnPQ+x;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=sourceware.org\n (client-ip=2620:52:6:3111::32; helo=vm01.sourceware.org;\n envelope-from=libc-alpha-bounces~incoming=patchwork.ozlabs.org@sourceware.org;\n receiver=patchwork.ozlabs.org)","sourceware.org;\n\tdkim=pass (2048-bit key,\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=QBOnPQ+x","sourceware.org;\n dmarc=pass (p=none dis=none) header.from=gmail.com","sourceware.org; spf=pass smtp.mailfrom=gmail.com","server2.sourceware.org;\n arc=pass smtp.remote-ip=2a00:1450:4864:20::631"],"Received":["from vm01.sourceware.org (vm01.sourceware.org\n [IPv6:2620:52:6:3111::32])\n\t(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)\n\t key-exchange x25519 server-signature ECDSA (secp384r1) server-digest SHA384)\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4g15sW1StMz1yGs\n\tfor ; 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Wed, 22 Apr 2026\n 10:32:38 -0700 (PDT)","MIME-Version":"1.0","References":"<20260422164926.62177-1-marocketbd@gmail.com>\n <874il3lyig.fsf@gmail.com>","In-Reply-To":"<874il3lyig.fsf@gmail.com>","From":"Rocket Ma ","Date":"Thu, 23 Apr 2026 01:32:23 +0800","X-Gm-Features":"AQROBzDhVr3XxySP4rih1zB4fiOhsT7I47voSPlZLR7jpnWAww2VtFCbmTu5QSo","Message-ID":"\n ","Subject":"Re: [PATCH v2] misc: Optimize getusershell.c","To":"Collin Funk ","Cc":"libc-alpha@sourceware.org","Content-Type":"text/plain; charset=\"UTF-8\"","X-BeenThere":"libc-alpha@sourceware.org","X-Mailman-Version":"2.1.30","Precedence":"list","List-Id":"Libc-alpha mailing list ","List-Unsubscribe":",\n ","List-Archive":"","List-Post":"","List-Help":"","List-Subscribe":",\n ","Errors-To":"libc-alpha-bounces~incoming=patchwork.ozlabs.org@sourceware.org"}},{"id":3681257,"web_url":"http://patchwork.ozlabs.org/comment/3681257/","msgid":"<874il2l6tc.fsf@gmail.com>","list_archive_url":null,"date":"2026-04-23T03:00:15","subject":"Re: [PATCH v2] misc: Optimize getusershell.c","submitter":{"id":88687,"url":"http://patchwork.ozlabs.org/api/people/88687/","name":"Collin Funk","email":"collin.funk1@gmail.com"},"content":"Adhemerval Zanella Netto writes:\n\n> On 22/04/26 14:01, Collin Funk wrote:\n>> Rocket Ma writes:\n>> \n>>> * misc/getusershell.c: Completely rewrite the unit. Only allocate one\n>>> big buffer to store shell names. Add a missing unit test.\n>>>\n>>> The new implementation read the whole file into one buffer, and wipe out\n>>> every byte but shell names. Later when addressing shell names from first\n>>> shell, jump to next '\\0' and then jump to next '/'. This could reduce\n>>> memory footprint and shall improve some performance.\n>>>\n>>> Signed-off-by: Rocket Ma \n>>> ---\n>>> This new patch fixed some unexpected plt links, and use isspace_l to\n>>> force parsing /etc/shells with C locale. (Is that necessary?)\n>> \n>> Thanks for the patch.\n>> \n>> However, I am against this change. The /etc/shells file can be\n>> arbitrarily long, so we shouldn't read it all into memory. It also\n>> changes the behavior of the function to be different to all the BSD\n>> versions. In this version appending a shell while the process is running\n>> will not affect subsequent calls to getusershell.\n>\n> The current code already read the whole file before parsing it, this\n> new approach is no worse. The BSD code also seems to limit each\n> shell to PATH_MAX, while this approach seems to work in-place and\n> removes the extra buffer allocation.\n\nAh, thanks to you and Rocket for pointing out my mistake. I think I was\nmixing up the one in glibc and the one I wrote for Gnulib to work around\nsome issues. Sorry for the extra noise.\n\nThe one in Gnulib uses getline, mostly because it was easier. I guess\nthe downside to that is that you might read a very long line which would\nbe impossible to execute as a shell, instead of limiting things to\nPATH_MAX. It looks like OpenBSD decided to go in that direction as well\n[1].\n\nCollin\n\n[1] https://github.com/openbsd/src/blob/7a2e62a4900cc0c671c264dfa2679f73987d45c6/lib/libc/gen/getusershell.c#L103","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","libc-alpha@sourceware.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","libc-alpha@sourceware.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=jbwOJcB/;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=sourceware.org\n (client-ip=2620:52:6:3111::32; helo=vm01.sourceware.org;\n envelope-from=libc-alpha-bounces~incoming=patchwork.ozlabs.org@sourceware.org;\n receiver=patchwork.ozlabs.org)","sourceware.org;\n\tdkim=pass (2048-bit key,\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=jbwOJcB/","sourceware.org;\n dmarc=pass (p=none dis=none) header.from=gmail.com","sourceware.org; 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I think I was\n> mixing up the one in glibc and the one I wrote for Gnulib to work around\n> some issues. Sorry for the extra noise.\n>\n> The one in Gnulib uses getline, mostly because it was easier. I guess\n> the downside to that is that you might read a very long line which would\n> be impossible to execute as a shell, instead of limiting things to\n> PATH_MAX. It looks like OpenBSD decided to go in that direction as well\n> [1].\n>\n> Collin\n>\n> [1] https://github.com/openbsd/src/blob/7a2e62a4900cc0c671c264dfa2679f73987d45c6/lib/libc/gen/getusershell.c#L103\n\nThis patch aims at reducing memory footprint, since it only allocates\none big buffer, then frees it. The old implementation allocates a\nfile-size buffer to store string and (file size / 3 * 8) to store\npointers. The openbsd implementation you pointed out uses getline +\nstrdup with a dynamic array to allocate strings and string pointers,\nleading to multiple malloc calls. For glibc, only allocate one big\nbuffer could help malloc to trim or split it later should improve\nperformance, rather than leave a lot of free chunks that may never be\nused in bins.\n\nRocket","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","libc-alpha@sourceware.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","libc-alpha@sourceware.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=UATldQmt;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=sourceware.org\n (client-ip=2620:52:6:3111::32; helo=vm01.sourceware.org;\n envelope-from=libc-alpha-bounces~incoming=patchwork.ozlabs.org@sourceware.org;\n receiver=patchwork.ozlabs.org)","sourceware.org;\n\tdkim=pass (2048-bit key,\n unprotected) header.d=gmail.com header.i=@gmail.com header.a=rsa-sha256\n header.s=20251104 header.b=UATldQmt","sourceware.org;\n dmarc=pass (p=none dis=none) header.from=gmail.com","sourceware.org; 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