proof for dlmem() (Re: [PATCH v9 0/13] implement dlmem() function)

[stsp <stsp2@yandex.ru>]

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Hello,

14.04.2023 01:02, Adhemerval Zanella Netto пишет:
> "It would be possible to require the caller to arrange all of
> these things, but that's basically offloading A LOT of the ELF
> loading process onto the calling program and I don't think that
> makes for a reasonable public interface for glibc to provide."
OK, in this case I am going to provide
a very detailed, reproducible and undisputable
proof that the above quote is false.

Attached is the small patch that can be
applied on top of my v10 patch-set to verify
the demo I present here.

The demo I present here, shows the "trace"
of the dlmem() usage, by sampling the
/proc/self/maps at needed points. Namely,
before dlmem(), immediately after dlmem(),
and after the fdlopen-over-dlmem finished.

That demo clearly and unambiguously demonstrates
how dlmem() transforms the raw solib image
into the "loaded" image, i.e. where all the
PT_LOAD sections are correctly laid out.

The output of that demo will be located in
a path: <glibc-build-dir>/dlfcn/tst-dlmem-extfns.out
Everyone can apply the v10+attached patch
and verify my results on his build.

And here it is:

$ cat tst-dlmem-extfns.out

before dlmem
7f5210ca8000-7f5210cad000 r--p 00000000 00:29 18840304 
/home/stas/src/glibc-dlmem/build/dlfcn/glreflib1.so

after dlmem
7f5210ca3000-7f5210ca4000 r--p 00000000 00:29 18840304 
/home/stas/src/glibc-dlmem/build/dlfcn/glreflib1.so
7f5210ca4000-7f5210ca5000 r-xp 00001000 00:29 18840304 
/home/stas/src/glibc-dlmem/build/dlfcn/glreflib1.so
7f5210ca5000-7f5210ca6000 r--p 00002000 00:29 18840304 
/home/stas/src/glibc-dlmem/build/dlfcn/glreflib1.so
7f5210ca6000-7f5210ca7000 r--p 00002000 00:29 18840304 
/home/stas/src/glibc-dlmem/build/dlfcn/glreflib1.so
7f5210ca7000-7f5210ca8000 rw-p 00003000 00:29 18840304 
/home/stas/src/glibc-dlmem/build/dlfcn/glreflib1.so
7f5210ca8000-7f5210cad000 r--p 00000000 00:29 18840304 
/home/stas/src/glibc-dlmem/build/dlfcn/glreflib1.so

post fdlopen
7f5210ca3000-7f5210ca4000 r--p 00000000 00:29 18840304 
/home/stas/src/glibc-dlmem/build/dlfcn/glreflib1.so
7f5210ca4000-7f5210ca5000 r-xp 00001000 00:29 18840304 
/home/stas/src/glibc-dlmem/build/dlfcn/glreflib1.so
7f5210ca5000-7f5210ca6000 r--p 00002000 00:29 18840304 
/home/stas/src/glibc-dlmem/build/dlfcn/glreflib1.so
7f5210ca6000-7f5210ca7000 r--p 00002000 00:29 18840304 
/home/stas/src/glibc-dlmem/build/dlfcn/glreflib1.so
7f5210ca7000-7f5210ca8000 rw-p 00003000 00:29 18840304 
/home/stas/src/glibc-dlmem/build/dlfcn/glreflib1.so


PS: yes, I realize nothing can be changed.
But I need to make sure the best possible and
the most obvious proof is publically available,
so that its not to look as if I've given up to
this "wonderful argument that was brought
to me so many times". You got all the proves,
and also more than once.

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diff --git a/dlfcn/tst-dlmem-extfns.c b/dlfcn/tst-dlmem-extfns.c
index 3b98a6e859..e35bdcaeb1 100644
--- a/dlfcn/tst-dlmem-extfns.c
+++ b/dlfcn/tst-dlmem-extfns.c
@@ -36,13 +36,20 @@ fdlopen (int fd, int flags)
   off_t len;
   void *addr;
   void *handle;
+  char cmd[256];
 
   len = lseek (fd, 0, SEEK_END);
   lseek (fd, 0, SEEK_SET);
   addr = mmap (NULL, len, PROT_READ, MAP_PRIVATE, fd, 0);
   if (addr == MAP_FAILED)
     return NULL;
+  printf ("\nbefore dlmem\n");
+  snprintf (cmd, sizeof(cmd), "grep glreflib1.so /proc/%i/maps", getpid());
+  system (cmd);
   handle = dlmem (addr, len, flags, NULL);
+  printf ("\nafter dlmem\n");
+  snprintf (cmd, sizeof(cmd), "grep glreflib1.so /proc/%i/maps", getpid());
+  system (cmd);
   munmap (addr, len);
   return handle;
 }
@@ -111,8 +118,8 @@ do_test (void)
   handle = dlmem (unaligned_buf, 4096, RTLD_NOW, NULL);
   TEST_VERIFY (handle == NULL);
   /* errno is set by dlerror() so needs to print something. */
-  printf ("unaligned buf gives %s\n", dlerror ());
-  TEST_COMPARE (errno, EINVAL);
+//  printf ("unaligned buf gives %s\n", dlerror ());
+//  TEST_COMPARE (errno, EINVAL);
 
   fd = open (BUILDDIR "glreflib1.so", O_RDONLY);
   if (fd == -1)
@@ -128,10 +135,11 @@ do_test (void)
 
   /* Check that the lib is properly mmap()ed, rather than memcpy()ed.
      This may fail on linux kernels <5.13. */
+  printf ("\npost fdlopen\n");
   snprintf (cmd, sizeof(cmd), "grep glreflib1.so /proc/%i/maps", getpid());
   rc = system (cmd);
   TEST_COMPARE (rc, 0);
-
+return 0;
   sym = dlsym (handle, "ref1");
   if (sym == NULL)
     error (EXIT_FAILURE, 0, "dlsym failed");