[Minios-devel] [UNIKRAFT PATCH 2/3] lib/syscall_shim: Option to disable libc-style wrapper functions

[Simon Kuenzer <simon.kuenzer@xxxxxxxxx>]

Introduces the hidden Config.uk option `LIBSYSCALL_SHIM_NOWRAPPER` to
disable the generation of libc-style syscall wrapper functions. The
idea is that libC libraries (e.g., musl) that provide own
wrappers (e.g., based on `uk_syscall()` and `uk_syscall_static()`)
would set this option through their Config.uk. In such a case, the
syscall_shim library will then only generate the low-level system call
implementations.

Signed-off-by: Simon Kuenzer <simon.kuenzer@xxxxxxxxx>
---
 doc/guides/developers-app.rst         |  9 +++++++
 lib/syscall_shim/Config.uk            |  6 +++++
 lib/syscall_shim/include/uk/syscall.h | 35 +++++++++++++++++++++++++++
 3 files changed, 50 insertions(+)

diff --git a/doc/guides/developers-app.rst b/doc/guides/developers-app.rst
index 2e6e7757..f14ed5a2 100644
--- a/doc/guides/developers-app.rst
+++ b/doc/guides/developers-app.rst
@@ -441,11 +441,20 @@ libc-style wrapper on top:
         return ret;
     }
 
+    #if UK_LIBC_SYSCALL
     ssize_t write(int fd, const void *buf, size_t count)
     {
         return (ssize_t) uk_syscall_e_write((long) fd,
                                             (long) buf, (long) count);
     }
+    #endif /* UK_LIBC_SYSCALL */
+
+Note: Please note that the implementation of custom libc-style wrappers have to
+be guarded with ``#if UK_LIBC_SYSCALL``. This macro is provided by the
+``<uk/syscall.h>`` header. Some libC ports (e.g., musl) deactivate this option
+whenever their provide own wrapper functions. For such cases, the syscall_shim
+library will only provide the ``uk_syscall_e_<syscall_name>`` and
+``uk_syscall_r_<syscall_name>`` symbols.
 
 Note: When `syscall_shim` library is not enabled, the original design idea was
 that the macros provide the libc-style wrapper only. However, all the
diff --git a/lib/syscall_shim/Config.uk b/lib/syscall_shim/Config.uk
index 20962590..c3369463 100644
--- a/lib/syscall_shim/Config.uk
+++ b/lib/syscall_shim/Config.uk
@@ -3,6 +3,12 @@ menuconfig LIBSYSCALL_SHIM
        default n
 
 if LIBSYSCALL_SHIM
+       # Hidden configuration option that can be set by libc's in order to
+       # switch off the generation of libc-style wrapper symbols
+       config LIBSYSCALL_SHIM_NOWRAPPER
+       bool
+       default n
+
        config LIBSYSCALL_SHIM_HANDLER
                bool "Binary system call handler (Linux ABI)"
                default n
diff --git a/lib/syscall_shim/include/uk/syscall.h 
b/lib/syscall_shim/include/uk/syscall.h
index a219365b..6ee139fa 100644
--- a/lib/syscall_shim/include/uk/syscall.h
+++ b/lib/syscall_shim/include/uk/syscall.h
@@ -42,6 +42,20 @@
 #include <errno.h>
 #include <uk/print.h>
 
+/*
+ * Whenever the hidden Config.uk option LIBSYSCALL_SHIM_NOWRAPPER
+ * is set, the creation of libc-style wrappers are disable by the
+ * UK_SYSCALL_DEFINE() and UK_SYSCALL_R_DEFINE() macros. Alternatively,
+ * UK_LIBC_SYSCALLS can be set to 0 through compilation flags.
+ */
+#ifndef UK_LIBC_SYSCALLS
+#if CONFIG_LIBSYSCALL_SHIM_NOWRAPPER
+#define UK_LIBC_SYSCALLS (0)
+#else
+#define UK_LIBC_SYSCALLS (1)
+#endif /* CONFIG_LIBSYSCALL_SHIM_NOWRAPPER */
+#endif /* UK_LIBC_SYSCALLS */
+
 #define __uk_scc(X) ((long) (X))
 typedef long uk_syscall_arg_t;
 
@@ -135,7 +149,9 @@ typedef long uk_syscall_arg_t;
 /*
  * UK_SYSCALL_DEFINE()
  * Based on UK_LLSYSCALL_DEFINE and provides a libc-style wrapper
+ * in case UK_LIBC_SYSCALLS is enabled
  */
+#if UK_LIBC_SYSCALLS
 #define __UK_SYSCALL_DEFINE(x, rtype, name, ename, rname, ...)         \
        long ename(UK_ARG_MAPx(x, UK_S_ARG_LONG, __VA_ARGS__));         \
        rtype name(UK_ARG_MAPx(x, UK_S_ARG_ACTUAL, __VA_ARGS__))        \
@@ -152,6 +168,15 @@ typedef long uk_syscall_arg_t;
                           __UK_NAME2SCALLE_FN(name),                   \
                           __UK_NAME2SCALLR_FN(name),                   \
                           __VA_ARGS__)
+#else
+#define UK_SYSCALL_DEFINE(rtype, name, ...)                            \
+       _UK_LLSYSCALL_DEFINE(__UK_SYSCALL_DEF_NARGS(__VA_ARGS__),       \
+                            rtype,                                     \
+                            name,                                      \
+                            __UK_NAME2SCALLE_FN(name),                 \
+                            __UK_NAME2SCALLR_FN(name),                 \
+                            __VA_ARGS__)
+#endif /* UK_LIBC_SYSCALLS */
 
 /* Raw system call implementation that is returning negative codes on errors */
 /* TODO: `void` as return type is currently not supported.
@@ -194,7 +219,9 @@ typedef long uk_syscall_arg_t;
 /*
  * UK_SYSCALL_R_DEFINE()
  * Based on UK_LLSYSCALL_R_DEFINE and provides a libc-style wrapper
+ * in case UK_LIBC_SYSCALLS is enabled
  */
+#if UK_LIBC_SYSCALLS
 #define __UK_SYSCALL_R_DEFINE(x, rtype, name, ename, rname, ...)       \
        long ename(UK_ARG_MAPx(x, UK_S_ARG_LONG, __VA_ARGS__));         \
        rtype name(UK_ARG_MAPx(x, UK_S_ARG_ACTUAL, __VA_ARGS__))        \
@@ -211,6 +238,14 @@ typedef long uk_syscall_arg_t;
                             __UK_NAME2SCALLE_FN(name),                 \
                             __UK_NAME2SCALLR_FN(name),                 \
                             __VA_ARGS__)
+#else
+#define UK_SYSCALL_R_DEFINE(rtype, name, ...)                          \
+       _UK_LLSYSCALL_R_DEFINE(__UK_SYSCALL_DEF_NARGS(__VA_ARGS__),     \
+                              name,                                    \
+                              __UK_NAME2SCALLE_FN(name),               \
+                              __UK_NAME2SCALLR_FN(name),               \
+                              __VA_ARGS__)
+#endif /* UK_LIBC_SYSCALLS */
 
 
 #define __UK_SPROTO_ARGS_TYPE long
-- 
2.20.1


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