perf tools: Enable reading of perf.data files from different ABI rev

This patch allows perf to process perf.data files generated
using an ABI that has a different perf_event_attr struct size,
i.e., a different ABI version.

The perf_event_attr can be extended, yet perf needs to cope with
older perf.data files. Similarly, perf must be able to cope with
a perf.data file which is using a newer version of the ABI than
what it knows about.

This patch adds read_attr(), a routine that reads a
perf_event_attr struct from a file incrementally based on its
advertised size. If the on-file struct is smaller than what perf
knows, then the extra fields are zeroed. If the on-file struct
is bigger, then perf only uses what it knows about, the rest is
skipped.

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: peterz@infradead.org
Cc: acme@redhat.com
Cc: robert.richter@amd.com
Cc: ming.m.lin@intel.com
Cc: andi@firstfloor.org
Cc: asharma@fb.com
Cc: ravitillo@lbl.gov
Cc: vweaver1@eecs.utk.edu
Cc: khandual@linux.vnet.ibm.com
Cc: dsahern@gmail.com
Link: http://lkml.kernel.org/r/1328826068-11713-17-git-send-email-eranian@google.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 50 insertions, 7 deletions

diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index 9f867d96c6a5..666f18972fa3 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -1973,6 +1973,51 @@ static int perf_header__read_pipe(struct perf_session *session, int fd)
         return 0;
 }
 
+static int read_attr(int fd, struct perf_header *ph,
+                     struct perf_file_attr *f_attr)
+{
+        struct perf_event_attr *attr = &f_attr->attr;
+        size_t sz, left;
+        size_t our_sz = sizeof(f_attr->attr);
+        int ret;
+
+        memset(f_attr, 0, sizeof(*f_attr));
+
+        /* read minimal guaranteed structure */
+        ret = readn(fd, attr, PERF_ATTR_SIZE_VER0);
+        if (ret <= 0) {
+                pr_debug("cannot read %d bytes of header attr\n",
+                         PERF_ATTR_SIZE_VER0);
+                return -1;
+        }
+
+        /* on file perf_event_attr size */
+        sz = attr->size;
+        if (ph->needs_swap)
+                sz = bswap_32(sz);
+
+        if (sz == 0) {
+                /* assume ABI0 */
+                sz =  PERF_ATTR_SIZE_VER0;
+        } else if (sz > our_sz) {
+                pr_debug("file uses a more recent and unsupported ABI"
+                         " (%zu bytes extra)\n", sz - our_sz);
+                return -1;
+        }
+        /* what we have not yet read and that we know about */
+        left = sz - PERF_ATTR_SIZE_VER0;
+        if (left) {
+                void *ptr = attr;
+                ptr += PERF_ATTR_SIZE_VER0;
+
+                ret = readn(fd, ptr, left);
+        }
+        /* read perf_file_section, ids are read in caller */
+        ret = readn(fd, &f_attr->ids, sizeof(f_attr->ids));
+
+        return ret <= 0 ? -1 : 0;
+}
+
 int perf_session__read_header(struct perf_session *session, int fd)
 {
         struct perf_header *header = &session->header;
@@ -1988,19 +2033,17 @@ int perf_session__read_header(struct perf_session *session, int fd)
         if (session->fd_pipe)
                 return perf_header__read_pipe(session, fd);
 
-        if (perf_file_header__read(&f_header, header, fd) < 0) {
-                pr_debug("incompatible file format\n");
+        if (perf_file_header__read(&f_header, header, fd) < 0)
                 return -EINVAL;
-        }
 
-        nr_attrs = f_header.attrs.size / sizeof(f_attr);
+        nr_attrs = f_header.attrs.size / f_header.attr_size;
         lseek(fd, f_header.attrs.offset, SEEK_SET);
 
         for (i = 0; i < nr_attrs; i++) {
                 struct perf_evsel *evsel;
                 off_t tmp;
 
-                if (readn(fd, &f_attr, sizeof(f_attr)) <= 0)
+                if (read_attr(fd, header, &f_attr) < 0)
                         goto out_errno;
 
                 if (header->needs_swap)
diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index bec8a328b1b8..e650de8f4396 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -24,7 +24,7 @@ static int perf_session__open(struct perf_session *self, bool force)
                 self->fd = STDIN_FILENO;
 
                 if (perf_session__read_header(self, self->fd) < 0)
-                        pr_err("incompatible file format");
+                        pr_err("incompatible file format (rerun with -v to learn more)");
 
                 return 0;
         }
@@ -56,7 +56,7 @@ static int perf_session__open(struct perf_session *self, bool force)
         }
 
         if (perf_session__read_header(self, self->fd) < 0) {
-                pr_err("incompatible file format");
+                pr_err("incompatible file format (rerun with -v to learn more)");
                 goto out_close;
         }