[2019-06] [android] mono_dl_open_file: use g_file_test only on absolute paths #16392
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The intention of calling `g_file_test (file, G_FILE_TEST_EXISTS)` where file is the name of a shared library we want to open is to speed up probing for non-existent libraries. See mono#12074 The problem is that if file is just a simple "libdl.so" then `dlopen (file)` doesn't just look for it in the current working directory, it will probe some other paths too. (For example on desktop linux you'd also look in all the directories in LD_LIBRARY_PATH). So the g_file_test() call is not a robust way to avoid calling dlopen if the filename is relative. But it actually broke more things: dotnet/android#3388 When probing for "libdl.so" on Android mono_lookup_pinvoke_call will first try prepending some paths that it knows about and we end up calling `dlopen ("/system/lib/libdl.so")` which will fail because Bionic has security restrictions on what code can dlopen something from /system/lib with an absolute path. Eventually mono_lookup_pinvoke_call will go back to trying the bare "libdl.so" which hits `g_file_test` and returns NULL. The new code only does the file test if we pass it an absolute path, which gives Bionic's dlopen a chance to deal with relative paths however it needs to.
lambdageek
approved these changes
Aug 21, 2019
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The intention of calling
g_file_test (file, G_FILE_TEST_EXISTS)where file isthe name of a shared library we want to open is to speed up probing for
non-existent libraries.
See #12074
The problem is that if file is just a simple "libdl.so" then
dlopen (file)doesn't just look for it in the current working directory, it will probe some
other paths too. (For example on desktop linux you'd also look in all the
directories in LD_LIBRARY_PATH). So the g_file_test() call is not a robust way
to avoid calling dlopen if the filename is relative.
But it actually broke more things: dotnet/android#3388
When probing for "libdl.so" on Android mono_lookup_pinvoke_call will first try
prepending some paths that it knows about and we end up calling
dlopen ("/system/lib/libdl.so")which will fail because Bionic has securityrestrictions on what code can dlopen something from /system/lib with an
absolute path. Eventually mono_lookup_pinvoke_call will go back to trying the
bare "libdl.so" which hits
g_file_testand returns NULL.The new code only does the file test if we pass it an absolute path, which
gives Bionic's dlopen a chance to deal with relative paths however it needs to.
Addresses dotnet/android#3388
Backport of #16387.
/cc @lambdageek