libdrgn: Use DRGN_VMCOREINFO var as an override

Sometimes vmcores don't contain vmcoreinfo, or don't contain the correct
vmcoreinfo. Add logic to use the DRGN_VMCOREINFO variable to override
what is (or is not) found in the ELF notes, or found by libkdumpfile.
This can help open broken vmcores, such as strange hypervisor vmcores.

Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>

docs/advanced_usage.rst

@@ -120,6 +120,16 @@ Some of drgn's behavior can be modified through environment variables:
     dump (0 or 1). The default is 1. This environment variable is mainly
     intended for testing and may be ignored in the future.
 
+``DRGN_VMCOREINFO``
+    Provide an alternative vmcoreinfo which overrides the one present (if any)
+    in the program itself. This can be useful for enabling drgn to load programs
+    which it could not ordinarily open. For example, if a vmcore was created by
+    a hypervisor which does not support the vmcoreinfo device (or it was not
+    enabled), then the vmcore itself may not contain the note, or worse: it may
+    contain a made-up note. By inspecting the vmcore using ``strings`` or other
+    custom tools, you may be able to find the vmcoreinfo note from within the
+    dump. In that case, you can provide this data via this environment variable.
+
 .. _kernel-special-objects:
 
 Linux Kernel Special Objects
@@ -203,10 +213,11 @@ core dumps. These special objects include:
 
     In the linux kernel, this data is normally stored in a variable called
     ``vmcoreinfo_data``. However, drgn reads this information from ELF note or
-    from the diskdump header. It is possible (in rare cases, usually with
-    vmcores created by hypervisors) for a vmcore to contain vmcoreinfo which
-    differs from the data in ``vmcoreinfo_data``, so it is important to
-    distinguish the contents. For that reason, we use the name ``VMCOREINFO`` to
-    distinguish it from the kernel variable ``vmcoreinfo_data``.
+    from the diskdump header. It is also possible for this information to be set
+    via the ``DRGN_VMCOREINFO`` variable. Thus, it is possible for a vmcore to
+    contain vmcoreinfo which differs from the data in ``vmcoreinfo_data``, so it
+    is important to distinguish the contents. For that reason, we use the name
+    ``VMCOREINFO`` to distinguish it from the kernel variable
+    ``vmcoreinfo_data``.
 
     This is available without debugging information.

libdrgn/kdump.c

@@ -108,28 +108,34 @@ struct drgn_error *drgn_program_set_kdump(struct drgn_program *prog)
 		goto err;
 	}
 
+	char *vmcoreinfo_override = getenv("DRGN_VMCOREINFO");
+	if (vmcoreinfo_override) {
+		err = drgn_program_parse_vmcoreinfo(prog, vmcoreinfo_override,
+						    strlen(vmcoreinfo_override) + 1);
+	} else {
 #if KDUMPFILE_VERSION >= KDUMPFILE_MKVER(0, 4, 1)
-	char *vmcoreinfo;
+		char *vmcoreinfo;
 #else
-	const char *vmcoreinfo;
+		const char *vmcoreinfo;
 #endif
-	ks = kdump_vmcoreinfo_raw(ctx, &vmcoreinfo);
-	if (ks != KDUMP_OK) {
-		err = drgn_error_format(DRGN_ERROR_OTHER,
-					"kdump_vmcoreinfo_raw: %s",
-					kdump_get_err(ctx));
-		goto err;
-	}
+		ks = kdump_vmcoreinfo_raw(ctx, &vmcoreinfo);
+		if (ks != KDUMP_OK) {
+			err = drgn_error_format(DRGN_ERROR_OTHER,
+						"kdump_vmcoreinfo_raw: %s",
+						kdump_get_err(ctx));
+			goto err;
+		}
 
-	err = drgn_program_parse_vmcoreinfo(prog, vmcoreinfo,
-					    strlen(vmcoreinfo) + 1);
-	/*
-	 * As of libkdumpfile 0.4.1, the string returned by
-	 * kdump_vmcoreinfo_raw() needs to be freed.
-	 */
+		err = drgn_program_parse_vmcoreinfo(prog, vmcoreinfo,
+						strlen(vmcoreinfo) + 1);
+		/*
+		* As of libkdumpfile 0.4.1, the string returned by
+		* kdump_vmcoreinfo_raw() needs to be freed.
+		*/
 #if KDUMPFILE_VERSION >= KDUMPFILE_MKVER(0, 4, 1)
-	free(vmcoreinfo);
+		free(vmcoreinfo);
 #endif
+	}
 	if (err)
 		goto err;
 

libdrgn/program.c

@@ -240,6 +240,10 @@ drgn_program_set_core_dump(struct drgn_program *prog, const char *path)
 	if (err)
 		return err;
 
+	vmcoreinfo_note = getenv("DRGN_VMCOREINFO");
+	if (vmcoreinfo_note)
+		vmcoreinfo_size = strlen(vmcoreinfo_note) + 1;
+
 	prog->core_fd = open(path, O_RDONLY);
 	if (prog->core_fd == -1)
 		return drgn_error_create_os("open", errno, path);
@@ -340,7 +344,8 @@ drgn_program_set_core_dump(struct drgn_program *prog, const char *path)
 						    HOST_LITTLE_ENDIAN)
 							bswap_64(prog->aarch64_insn_pac_mask);
 					}
-				} else if (nhdr.n_namesz == sizeof("VMCOREINFO") &&
+				} else if (!vmcoreinfo_note &&
+					   nhdr.n_namesz == sizeof("VMCOREINFO") &&
 					   memcmp(name, "VMCOREINFO",
 						  sizeof("VMCOREINFO")) == 0) {
 					vmcoreinfo_note = desc;