* Root directory being a junction fools realpath() @ 2021-02-14 11:56 David Macek 2021-02-15 12:25 ` Corinna Vinschen 0 siblings, 1 reply; 7+ messages in thread From: David Macek @ 2021-02-14 11:56 UTC (permalink / raw) To: cygwin-developers Greetings. I think I found a way to fool Cygwin into misplacing its root directory. Given recent Cygwin with GCC and a simple test program: ``` #include <stdio.h> #include <limits.h> #include <stdlib.h> int main() { char real[PATH_MAX]; realpath("/", real); printf("/ -> %s\n", real); return 0; } ``` ... compiled with: $ gcc -D_DEFAULT_SOURCE -o new.exe -Wall -Wextra -std=c11 new.c ... and my set-up with a junction (`mklink /j link target`) realpath() result is dependent on which path I use to invoke the Cygwin root process: > dir C:\ | findstr cygwin64 10. 02. 2021 12:13 <DIR> cygwin64 > dir D:\ | findstr cygwin64 14. 02. 2021 12:19 <JUNCTION> cygwin64 [C:\cygwin64] > C:\cygwin64\bin\bash -lc /cygdrive/w/new / -> / > D:\cygwin64\bin\bash -lc /cygdrive/w/new / -> /cygdrive/c/cygwin64 It seems the current working directory doesn't matter at all, same as the directory where the test program resides. Regular substs don't seem to have the same effect. I can do more tests if requested. I assume this is not intended behavior, because the root should always be the root for Cygwin processes, no matter how they're invoked. If possible ping me on IRC if anything because I don't check my mail too often. -- David Macek ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Root directory being a junction fools realpath() 2021-02-14 11:56 Root directory being a junction fools realpath() David Macek @ 2021-02-15 12:25 ` Corinna Vinschen 2021-02-15 13:20 ` Corinna Vinschen 0 siblings, 1 reply; 7+ messages in thread From: Corinna Vinschen @ 2021-02-15 12:25 UTC (permalink / raw) To: cygwin-developers On Feb 14 12:56, David Macek via Cygwin-developers wrote: > Greetings. > > I think I found a way to fool Cygwin into misplacing its root > directory. Given recent Cygwin with GCC and a simple test program: > > ``` > #include <stdio.h> > #include <limits.h> > #include <stdlib.h> > int main() { > char real[PATH_MAX]; > realpath("/", real); > printf("/ -> %s\n", real); > return 0; > } > ``` > > ... compiled with: > > $ gcc -D_DEFAULT_SOURCE -o new.exe -Wall -Wextra -std=c11 new.c > > ... and my set-up with a junction (`mklink /j link target`) realpath() > result is dependent on which path I use to invoke the Cygwin root > process: > > > dir C:\ | findstr cygwin64 > 10. 02. 2021 12:13 <DIR> cygwin64 > > > dir D:\ | findstr cygwin64 > 14. 02. 2021 12:19 <JUNCTION> cygwin64 [C:\cygwin64] > > > C:\cygwin64\bin\bash -lc /cygdrive/w/new > / -> / > > > D:\cygwin64\bin\bash -lc /cygdrive/w/new > / -> /cygdrive/c/cygwin64 This isn't realpath's fault. You're circumventing the mount point handling which is automated in terms of /, depending on the path returned from GetModuleFileNameW for the Cygwin DLL. Since you're calling D:\cygwin64\bin\bash the dir returned from GetModuleFileNameW is D:\cygwin64\bin, thus root is D:\cygwin64. However, junctions are treated as symlinks in Cygwin. Thus the result you see above. Corinna ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Root directory being a junction fools realpath() 2021-02-15 12:25 ` Corinna Vinschen @ 2021-02-15 13:20 ` Corinna Vinschen 2021-02-18 11:46 ` David Macek 0 siblings, 1 reply; 7+ messages in thread From: Corinna Vinschen @ 2021-02-15 13:20 UTC (permalink / raw) To: cygwin-developers On Feb 15 13:25, Corinna Vinschen via Cygwin-developers wrote: > On Feb 14 12:56, David Macek via Cygwin-developers wrote: > > Greetings. > > > > I think I found a way to fool Cygwin into misplacing its root > > directory. Given recent Cygwin with GCC and a simple test program: > > > > ``` > > #include <stdio.h> > > #include <limits.h> > > #include <stdlib.h> > > int main() { > > char real[PATH_MAX]; > > realpath("/", real); > > printf("/ -> %s\n", real); > > return 0; > > } > > ``` > > > > ... compiled with: > > > > $ gcc -D_DEFAULT_SOURCE -o new.exe -Wall -Wextra -std=c11 new.c > > > > ... and my set-up with a junction (`mklink /j link target`) realpath() > > result is dependent on which path I use to invoke the Cygwin root > > process: > > > > > dir C:\ | findstr cygwin64 > > 10. 02. 2021 12:13 <DIR> cygwin64 > > > > > dir D:\ | findstr cygwin64 > > 14. 02. 2021 12:19 <JUNCTION> cygwin64 [C:\cygwin64] > > > > > C:\cygwin64\bin\bash -lc /cygdrive/w/new > > / -> / > > > > > D:\cygwin64\bin\bash -lc /cygdrive/w/new > > / -> /cygdrive/c/cygwin64 > > This isn't realpath's fault. You're circumventing the mount point > handling which is automated in terms of /, depending on the path > returned from GetModuleFileNameW for the Cygwin DLL. Since you're > calling D:\cygwin64\bin\bash the dir returned from GetModuleFileNameW is > D:\cygwin64\bin, thus root is D:\cygwin64. > > However, junctions are treated as symlinks in Cygwin. Thus the result > you see above. A potential workaround is this: diff --git a/winsup/cygwin/cygheap.cc b/winsup/cygwin/cygheap.cc index 3a32c352d4e9..01b49468e804 100644 --- a/winsup/cygwin/cygheap.cc +++ b/winsup/cygwin/cygheap.cc @@ -144,6 +144,19 @@ init_cygheap::init_installation_root () api_fatal ("Can't initialize Cygwin installation root dir.\n" "GetModuleFileNameW(%p, %p, %u), %E", cygwin_hmodule, installation_root_buf, PATH_MAX); + + /* We don't care if fetching the final pathname fails, it's non-fatal and + the path returned by GetModuleFileNameW is still valid. */ + HANDLE h; + h = CreateFileW (installation_root_buf, GENERIC_READ, FILE_SHARE_VALID_FLAGS, + &sec_none, OPEN_EXISTING, 0, 0); + if (h != INVALID_HANDLE_VALUE) + { + GetFinalPathNameByHandleW (h, installation_root_buf, PATH_MAX, + FILE_NAME_NORMALIZED); + CloseHandle (h); + } + PWCHAR p = installation_root_buf; if (wcsncasecmp (p, L"\\\\", 2)) /* Normal drive letter path */ { I just wonder if such a bordercase is worth the extra performance hit for each Cygwin invocation. Corinna ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Root directory being a junction fools realpath() 2021-02-15 13:20 ` Corinna Vinschen @ 2021-02-18 11:46 ` David Macek 2021-02-18 13:09 ` Corinna Vinschen 0 siblings, 1 reply; 7+ messages in thread From: David Macek @ 2021-02-18 11:46 UTC (permalink / raw) To: cygwin-developers; +Cc: Corinna Vinschen Thank you for diagnosing. > > This isn't realpath's fault. You're circumventing the mount point > > handling It was not my intention. :) It's just due to my sharing scripts and configurations between various machines (with varying disk layouts and sizes), I sometimes need to use links to work around things. [...] > I just wonder if such a bordercase is worth the extra performance hit > for each Cygwin invocation. Of course I'd like to see it resolved, but I don't remember any non-cosmetic issue caused by it, *so far*. Is there an estimate of how big of a slow-down the fix causes? -- David Macek ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Root directory being a junction fools realpath() 2021-02-18 11:46 ` David Macek @ 2021-02-18 13:09 ` Corinna Vinschen 2021-02-19 18:24 ` Corinna Vinschen 0 siblings, 1 reply; 7+ messages in thread From: Corinna Vinschen @ 2021-02-18 13:09 UTC (permalink / raw) To: cygwin-developers On Feb 18 12:46, David Macek via Cygwin-developers wrote: > Thank you for diagnosing. > > > > This isn't realpath's fault. You're circumventing the mount point > > > handling > > It was not my intention. :) It's just due to my sharing scripts and > configurations between various machines (with varying disk layouts and > sizes), I sometimes need to use links to work around things. > > [...] > > > I just wonder if such a bordercase is worth the extra performance hit > > for each Cygwin invocation. > > Of course I'd like to see it resolved, but I don't remember any > non-cosmetic issue caused by it, *so far*. Is there an estimate of > how big of a slow-down the fix causes? No, somebody would have to test this. Probably it's not that much... Corinna ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Root directory being a junction fools realpath() 2021-02-18 13:09 ` Corinna Vinschen @ 2021-02-19 18:24 ` Corinna Vinschen 2021-03-07 13:15 ` David Macek 0 siblings, 1 reply; 7+ messages in thread From: Corinna Vinschen @ 2021-02-19 18:24 UTC (permalink / raw) To: cygwin-developers On Feb 18 14:09, Corinna Vinschen via Cygwin-developers wrote: > On Feb 18 12:46, David Macek via Cygwin-developers wrote: > > Thank you for diagnosing. > > > > > > This isn't realpath's fault. You're circumventing the mount point > > > > handling > > > > It was not my intention. :) It's just due to my sharing scripts and > > configurations between various machines (with varying disk layouts and > > sizes), I sometimes need to use links to work around things. > > > > [...] > > > > > I just wonder if such a bordercase is worth the extra performance hit > > > for each Cygwin invocation. > > > > Of course I'd like to see it resolved, but I don't remember any > > non-cosmetic issue caused by it, *so far*. Is there an estimate of > > how big of a slow-down the fix causes? > > No, somebody would have to test this. Probably it's not that much... I pushed the patch. New snapshots are in the works, should only take a couple of minutes. Corinna ^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Root directory being a junction fools realpath() 2021-02-19 18:24 ` Corinna Vinschen @ 2021-03-07 13:15 ` David Macek 0 siblings, 0 replies; 7+ messages in thread From: David Macek @ 2021-03-07 13:15 UTC (permalink / raw) To: cygwin-developers > I pushed the patch. New snapshots are in the works, should only take > a couple of minutes. > > Corinna I tested the patch in a local build and it seems to fix the issue. Thank you. -- David Macek ^ permalink raw reply [flat|nested] 7+ messages in thread
end of thread, other threads:[~2021-03-07 13:16 UTC | newest] Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2021-02-14 11:56 Root directory being a junction fools realpath() David Macek 2021-02-15 12:25 ` Corinna Vinschen 2021-02-15 13:20 ` Corinna Vinschen 2021-02-18 11:46 ` David Macek 2021-02-18 13:09 ` Corinna Vinschen 2021-02-19 18:24 ` Corinna Vinschen 2021-03-07 13:15 ` David Macek
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