Fix getprogname_linux.c for Android API < 21

This diff fixes the build of LibreSSL for Android with API < 21 where
the `getprogname` system call was not part of libc.

A comment in the diff itself explains in detail the issue.

1 files changed, 25 insertions, 0 deletions

diff --git a/crypto/compat/getprogname_linux.c b/crypto/compat/getprogname_linux.c
index fefe5ea..2c89743 100644
--- a/crypto/compat/getprogname_linux.c
+++ b/crypto/compat/getprogname_linux.c
@@ -5,5 +5,30 @@
 const char *
 getprogname(void)
 {
+        /*
+         * Android added getprogname with API 21 [0]. We should not end up here
+         * with APIs bigger than 21. Still write a precise check.
+         *
+         * Since Android is using portions of OpenBSD libc, it should have
+         * a symbol called __progname [1].
+         *
+         * Regarding program_invocation_short_name, it is a GNU libc ext [2] and
+         * so make it conditional to __GLIBC__ [3].
+         *
+         * .. [0] https://github.com/aosp-mirror/platform_bionic/blob/1eb6d3/libc/include/stdlib.h#L160
+         *
+         * .. [1] https://github.com/aosp-mirror/platform_bionic/commit/692207
+         *
+         * .. [2] https://linux.die.net/man/3/program_invocation_short_name
+         *
+         * .. [3] https://android.googlesource.com/platform/system/core/+/2819c0/base/logging.cpp#65
+         */
+#if defined(__ANDROID_API__) && __ANDROID_API__ < 21
+        extern const char *__progname;
+        return __progname;
+#elif defined(__GLIBC__)
         return program_invocation_short_name;
+#else
+#error "Cannot emulate getprogname"
+#endif
 }