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# MojoELF | ||
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MojoELF is an ELF binary loader that runs in your application instead of | ||
as part of the C runtime. The most useful feature of this is that, unlike | ||
the standard `dlopen()`, it can load an ELF file from a place other than | ||
the filesystem. Notably, it can load one from a buffer in memory. | ||
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## To use this nonsense: | ||
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- Add mojoelf.c to your build. | ||
- Compile with these #defines, or change the top of mojoelf.c: | ||
```c | ||
#define MOJOELF_SUPPORT_DLERROR 0 // remove MOJOELF_dlerror() + lots of strings. | ||
#define MOJOELF_SUPPORT_DLOPEN_FILE 0 // remove MOJOELF_dlopen_file() | ||
#define MOJOELF_REDUCE_LIBC_DEPENDENCIES 0 // use less libc calls. Scary! | ||
#define NDEBUG 1 // Turns off assert, which removes libc dependencies. | ||
``` | ||
- Your calling code should `#include mojoelf.h` ... | ||
- Put your ELF library in memory, and call `MOJOELF_dlopen_mem()` with the | ||
address of the memory buffer, the size of the buffer, and (optionally), | ||
callbacks that handle symbol resolution (they can be NULL). Details on | ||
the resolver callbacks are at the end of this document. | ||
- If `MOJOELF_dlopen_mem()` returns non-NULL, the library is ready to use. If | ||
it returns NULL, there was a problem (`MOJOELF_dlerror()` will give you a | ||
human-readable error message). On success, you can free your buffer; we | ||
don't need it after `MOJOELF_dlopen_mem()` returns. | ||
- `MOJOELF_dlopen_file()` does the same thing, but takes a filename instead of | ||
a memory buffer. Internally, it just loads the file into a malloc()'d | ||
buffer and calls `MOJOELF_dlopen_mem()`. | ||
- To request entry points into the library, use MOJOELF_dlsym(): | ||
```c | ||
int (*my_function)(int argument) = MOJOELF_dlsym(lib, "AwesomeFunc"); | ||
if (my_function == NULL) { | ||
printf("couldn't find AwesomeFunc!\n"); | ||
} else { | ||
printf("AwesomeFunc() returns %d\n", my_function(123)); | ||
} | ||
``` | ||
- When you are done with a library, call `MOJOELF_dlclose()` to free any | ||
resources. All pointers returned by `MOJOELF_dlsym()` for this library are | ||
invalid after this call. | ||
- Other fun stuff: `MOJOELF_getentry()` gets you the entry point for the | ||
ELF file (which isn't useful on shared libraries, but is how you eventually | ||
get to main() in an executable). | ||
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## Callbacks: | ||
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You supply a handful of callbacks when calling `MOJOELF_dlopen_*()`. Any given | ||
callback is allowed to be NULL, as is the pointer to the MOJOELF_callbacks if | ||
you don't care about these at all. The function pointers get copied, so you | ||
can free the MOJOELF_Callbacks struct as soon as `MOJOELF_dlopen_*()` returns. | ||
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```c | ||
typedef struct MOJOELF_Callbacks | ||
{ | ||
MOJOELF_LoaderCallback loader; | ||
MOJOELF_SymbolCallback resolver; | ||
MOJOELF_UnloaderCallback unloader; | ||
} MOJOELF_Callbacks; | ||
``` | ||
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Note that `MOJOELF_dlopen_*()` does not make any attempt to resolve dependencies | ||
on its own, so if you want to implement something like Linux's dynamic loader, | ||
you'll need to parse `LD_LIBRARY_PATH`, or whatever, on your own using these | ||
callbacks. By default, without callbacks, any dependency (including libc.so) | ||
or unresolved symbol will cause `MOJOELF_dlopen_*()` to fail. | ||
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The "loader" callback doesn't necessarily load anything. All it does it tell | ||
MojoELF that it's claiming a specific dependency. For example, if you want to | ||
load an ELF that depends on libFoo.so.3, but you plan to override this | ||
library without it actually existing, you can write a callback like this... | ||
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```c | ||
void *my_loader(const char *soname, const char *rpath, const char *runpath) | ||
{ | ||
return (void *) (strcmp(soname, "libFoo.so.3") == 0); | ||
} | ||
``` | ||
...and MojoELF will not try to load libFoo itself, and assumes you will | ||
provide any needed symbols from it via your resolver callback. | ||
Note that your loader can be way more complex...it could actually _load_ | ||
something, for example, but in many cases, this is all that's needed. | ||
The loader callback is provided with any `RPATH` or `RUNPATH` entries in the | ||
currently-loading ELF. Please refer to Linux's `dlopen()` manpage for details | ||
on these strings. | ||
The value returned from the loader callback is opaque data. It will be passed | ||
to your resolver callback, and is expected to be free'd in the unloader | ||
callback, if you like. | ||
The resolver callback looks like this: | ||
```c | ||
extern int my_function(int argument); | ||
void *my_resolver(void *handle, const char *sym) | ||
{ | ||
if (strcmp(sym, "my_function") == 0) | ||
return my_function; | ||
// this also works for data, not just functions. | ||
return NULL; /* can't help you. */ | ||
} | ||
``` | ||
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The callback is called once for each non-NULL value that your loader | ||
previously returned, in the other they were returned, until one succeeds. If | ||
none succeeds, the callback fires one more time with the handle set to NULL. | ||
If this still doesn't return a non-NULL value, MojoELF will fail to load the | ||
ELF file, due to missing dependencies. | ||
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The unloader callback is like this: | ||
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```c | ||
void my_unloader(void *handle) | ||
{ | ||
// if (handle) is something you allocated in your loader callback, | ||
// you can free it here. | ||
} | ||
``` | ||
## If you have problems: | ||
Ask Ryan: icculus@icculus.org | ||
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