Re: [musl] Invalid page size reference in __dls2

[Rich Felker <dalias@libc.org>]

[-- Attachment #1: Type: text/plain, Size: 1577 bytes --]

On Mon, Nov 28, 2022 at 08:47:40PM +0100, Markus Wichmann wrote:
> Hi all,
> 
> __dls2 calls kernel_mapped_dso(), and that one uses the PAGE_SIZE macro.
> Whenever <bits/limits.h> does not define PAGESIZE, PAGE_SIZE is defined
> as libc.page_size. That variable is only initialized at the start of
> __dls3, so the DSO descriptor for libc ends up being wrong.
> 
> Since the libc object has static storage duration, page_size is
> initialized with zero. So at least nothing undefined happens. The impact
> is, it will calculate the relro pointers as being zero, so no relro will
> happen, and it will calculate maximum and minimum addresses as being
> zero, therefore setting map to base and map_len to zero. This will cause
> dladdr() not to find the libc. Yeah, not the biggest of impacts.
> 
> This, again, affects all architectures that don't define PAGESIZE, so at
> this time those are
> 
> aarch64
> arm
> m68k
> microblaze
> mips
> mips64
> mipsn32
> powerpc
> powerpc64
> riscv64
> 
> I don't know whether references to libc are even valid in __dls2, but it
> is defined as "hidden", so that ought to be good enough. In that case it
> may be enough to just move the initialization. Otherwise it may be
> necessary to add page size as parameter to kernel_mapped_dso(). Then
> __dls2 can look it up in the aux vector at its leisure.

Nice catch. The references to libc are not valid in __dls2. If they
were, I would just re-run kernel_mapped_dso() from __dls2b or
something to get the right relro map, but I think instead we should do
something like the attached.

Rich

[-- Attachment #2: ldso_page_size.diff --]
[-- Type: text/plain, Size: 752 bytes --]

diff --git a/ldso/dynlink.c b/ldso/dynlink.c
index 8068fb37..fb13a7b1 100644
--- a/ldso/dynlink.c
+++ b/ldso/dynlink.c
@@ -21,9 +21,15 @@
 #include <sys/membarrier.h>
 #include "pthread_impl.h"
 #include "fork_impl.h"
-#include "libc.h"
 #include "dynlink.h"
 
+static size_t ldso_page_size;
+#ifndef PAGE_SIZE
+define PAGE_SIZE ldso_page_size;
+#endif
+
+#include "libc.h"
+
 #define malloc __libc_malloc
 #define calloc __libc_calloc
 #define realloc __libc_realloc
@@ -1723,6 +1729,7 @@ hidden void __dls2(unsigned char *base, size_t *sp)
 	ldso.phnum = ehdr->e_phnum;
 	ldso.phdr = laddr(&ldso, ehdr->e_phoff);
 	ldso.phentsize = ehdr->e_phentsize;
+	search_vec(auxv, &ldso_page_size, AT_PAGESZ);
 	kernel_mapped_dso(&ldso);
 	decode_dyn(&ldso);