[PATCH 3/3] Rewrite readpage_elf

The current code in readpage_elf (and readpage_elf_parallel) is extremely
hard to follow. Additionally, it still does not cover all possible cases.
For example, attempts to read outside of any ELF segment will end up with
phys_start being 0, frac_head a negative number, interpreted as a large
positive number by memset() and write past buffer end.

Instead of trying to handle even more "corner cases", I rewrote the
algorithm from scratch. The basic idea is simple: set a goal to fill the
page buffer with data, then work towards that goal by:

  - filling holes with zeroes (see Note below),
  - p_filesz portions with file data and
  - remaining p_memsz portions again with zeroes.

Repeat this method for each LOAD until the goal is achieved, or an error
occurs. In most cases, the loop runs only once.

Note: A "hole" is data at a physical address that is not covered by any
ELF LOAD program header. In other words, the ELF file does not specify
any data for such a hole (not even zeroes). So, why does makedumpfile
fill them with zeroes? It's because makedumpfile works with page
granularity (the compressed format does not even have a way to store
a partial page), so if only part of a page is stored, a complete page
must be provided to make this partial data accessible.

Credits to Ivan Delalande <colona@arista.com> who first found the
problem and wrote the original fix.

Tested-by: Ivan Delalande <colona@arista.com>
Signed-off-by: Petr Tesarik <ptesarik@suse.com>

1 files changed, 28 insertions, 0 deletions

diff --git a/elf_info.c b/elf_info.c
index 80a4da6..65ff333 100644
--- a/elf_info.c
+++ b/elf_info.c
@@ -691,6 +691,34 @@ get_max_paddr(void)
 	return max_paddr;
 }
 
+/*
+ * Find the LOAD segment which is closest to the requested
+ * physical address within a given distance.
+ *  If there is no such segment, return a negative number.
+ */
+int
+closest_pt_load(unsigned long long paddr, unsigned long distance)
+{
+	int i, bestidx;
+	struct pt_load_segment *pls;
+	unsigned long bestdist;
+
+	bestdist = distance;
+	bestidx = -1;
+	for (i = 0; i < num_pt_loads; ++i) {
+		pls = &pt_loads[i];
+		if (paddr >= pls->phys_end)
+			continue;
+		if (paddr >= pls->phys_start)
+			return i;	/* Exact match */
+		if (bestdist > pls->phys_start - paddr) {
+			bestdist = pls->phys_start - paddr;
+			bestidx = i;
+		}
+	}
+	return bestidx;
+}
+
 int
 get_elf64_ehdr(int fd, char *filename, Elf64_Ehdr *ehdr)
 {