Re: [PATCH v4 00/10] klp-convert livepatch build tooling

[Joe Lawrence <joe.lawrence@redhat.com>]

On Tue, Jun 25, 2019 at 01:36:37PM +0200, Miroslav Benes wrote:
>
> [ ... snip ... ]
>
> If I revert commit d59cadc0a8f8 ("[squash] klp-convert: make
> convert_rela() list-safe") (from Joe's expanded github tree), the problem
> disappears.
>
> I haven't spotted any problem in the code and I cannot explain a
> dependency on GCC version. Any ideas?
>

I can confirm that test_klp_convert1.ko crashes with RHEL-7 and its
older gcc.  I added some debugging printf's to klp-convert and see:

  % ./scripts/livepatch/klp-convert \
          ./Symbols.list \
          lib/livepatch/test_klp_convert1.klp.o \
          lib/livepatch/test_klp_convert1.ko | \
          grep saved_command_line

  convert_rela: oldsec: .rela.text rela @ 0x1279670 rela->sym @ 0x12791f0 (.klp.sym.vmlinux.saved_command_line,0) offset: 0x3
  convert_rela: oldsec: .rela.text rela @ 0x1279cd0 rela->sym @ 0x12791f0 (.klp.sym.vmlinux.saved_command_line,0) offset: 0x9a
  move_rela: rela @ 0x1279670 rela->sym @ 0x12791f0 (.klp.sym.vmlinux.saved_command_line,0) offset: 0x3
  main: skipping rela @ 0x1279cd0 rela->sym @ 0x12791f0 (.klp.sym.vmlinux.saved_command_line,0) (!must_convert)

I think the problem is:

- Relas at different offsets, but for the same symbol may share symbol
  storage.  Note the same rela->sym value above.

- Before d59cadc0a8f8 ("[squash] klp-convert: make convert_rela()
  list-safe"), convert_rela() iterated through the entire section's
  relas, moving any of the same name.  This was determined not to be
  list safe when moving consecutive relas in the linked list.

- After d59cadc0a8f8 ("[squash] klp-convert: make convert_rela()
  list-safe"), convert_rela() still iterates through the section relas,
  but only updates r1->sym->klp_rela_sec instead of moving them.
  move_rela() was added to be called by the for-each-rela loop in
  main().

  - Bug 1: klp_rela_sec probably belongs in struct rela and not struct
    symbol

  - Bug 2: the main loop skips over second, third, etc. matching relas
    anyway as the shared symbol name will have already been converted

The following fix might not be elegant, but I can't think of a clever
way to handle the original issue d59cadc0a8f8 ("[squash] klp-convert:
make convert_rela() list-safe") as well as these resulting regressions.
So I broke out the moving of relas to a seperate loop.  That is probably
worth a comment and at the same time we might be able to drop some of
these other "safe" loop traversals for ordinary list_for_each_entry.

-- Joe

-->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8-- -->8--

diff --git a/scripts/livepatch/elf.h b/scripts/livepatch/elf.h
index 7551409..57a8242 100644
--- a/scripts/livepatch/elf.h
+++ b/scripts/livepatch/elf.h
@@ -33,7 +33,6 @@ struct symbol {
 	struct list_head list;
 	GElf_Sym sym;
 	struct section *sec;
-	struct section *klp_rela_sec;
 	char *name;
 	unsigned int idx;
 	unsigned char bind, type;
@@ -45,6 +44,7 @@ struct rela {
 	struct list_head list;
 	GElf_Rela rela;
 	struct symbol *sym;
+	struct section *klp_rela_sec;
 	unsigned int type;
 	unsigned long offset;
 	int addend;
diff --git a/scripts/livepatch/klp-convert.c b/scripts/livepatch/klp-convert.c
index b5873ab..50d6471 100644
--- a/scripts/livepatch/klp-convert.c
+++ b/scripts/livepatch/klp-convert.c
@@ -525,7 +525,7 @@ static bool convert_rela(struct section *oldsec, struct rela *r,
 
 	list_for_each_entry_safe(r1, r2, &oldsec->relas, list) {
 		if (r1->sym->name == r->sym->name) {
-			r1->sym->klp_rela_sec = sec;
+			r1->klp_rela_sec = sec;
 		}
 	}
 	return true;
@@ -535,7 +535,7 @@ static void move_rela(struct rela *r)
 {
 	/* Move the converted rela to klp rela section */
 	list_del(&r->list);
-	list_add(&r->list, &r->sym->klp_rela_sec->relas);
+	list_add(&r->list, &r->klp_rela_sec->relas);
 }
 
 /* Checks if given symbol name matches a symbol in exp_symbols */
@@ -687,8 +687,11 @@ int main(int argc, const char **argv)
 					return -1;
 				}
 			}
+		}
 
-			move_rela(rela);
+		list_for_each_entry_safe(rela, tmprela, &sec->relas, list) {
+			if (is_converted(rela->sym->name))
+				move_rela(rela);
 		}
 	}