When running sandbox, the following phases occur, each with different malloc implementations or behaviors:
1) Dynamic linker execution, using the dynamic linker's own malloc() implementation. This is fully functional.
2) After U-Boot's malloc symbol has been hooked into the GOT, but before any U-Boot code has run. This phase is entirely non-functional, since U-Boot's gd symbol is NULL and U-Boot's initf_malloc() and mem_malloc_init() have not been called.
At least on Ubuntu Xenial, the dynamic linker does make both malloc() and free() calls during this phase. Currently these free() calls crash since they dereference gd, which is NULL.
U-Boot itself makes no use of malloc() during this phase.
3) U-Boot execution after gd is set and initf_malloc() has been called. This is fully functional, albeit via a very simple malloc() implementation.
4) U-Boot execution after mem_malloc_init() has been called. This is fully functional with a complete malloc() implementation.
Furthermore, if code that called malloc() during phase 1 calls free() in phase 3 or later, it is likely that heap corruption will occur, since U-Boot's malloc implementation will assume the pointer is part of its own heap, although it isn't. I have not actively observed this happening.
To prevent phase 2 from happening, this patch makes all of U-Boot's malloc library public symbols have hidden visibility. This prevents them from being hooked into the GOT, so only code in the U-Boot binary itself actually calls them; any other code will call into the standard C library malloc(). This also avoids the "furthermore" issue mentioned above.
I have seen references to this GCC pragma in blog posts from 2008, and RHEL5's ancient gcc appears to accept it fine, so I believe it's quite safe to use it without checking gcc version.
Cc: Rabin Vincent rabin@rab.in Signed-off-by: Stephen Warren swarren@wwwdotorg.org --- v2: A whole different approach. --- include/malloc.h | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/include/malloc.h b/include/malloc.h index f20e4d3d2a6b..8175c75920cf 100644 --- a/include/malloc.h +++ b/include/malloc.h @@ -914,6 +914,7 @@ int initf_malloc(void); /* Simple versions which can be used when space is tight */ void *malloc_simple(size_t size);
+#pragma GCC visibility push(hidden) # if __STD_C
Void_t* mALLOc(size_t); @@ -945,6 +946,7 @@ int mALLOPt(); struct mallinfo mALLINFo(); # endif #endif +#pragma GCC visibility pop
/* * Begin and End of memory area for malloc(), and current "brk"