[PATCH v1 1/2] perf symbol-minimal: Fix ehdr reading in filename__read_build_id

[Ian Rogers <irogers@google.com>]

The e_ident is part of the ehdr and so reading it a second time would
mean the read ehdr was displaced by 16-bytes. Switch from stdio to
open/read/lseek syscalls for similarity with the symbol-elf version of
the function and so that later changes can alter then open flags.

Fixes: fef8f648bb47 ("perf symbol: Fix use-after-free in filename__read_build_id")
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/util/symbol-minimal.c | 55 ++++++++++++++++----------------
 1 file changed, 27 insertions(+), 28 deletions(-)

diff --git a/tools/perf/util/symbol-minimal.c b/tools/perf/util/symbol-minimal.c
index 7201494c5c20..8d41bd7842df 100644
--- a/tools/perf/util/symbol-minimal.c
+++ b/tools/perf/util/symbol-minimal.c
@@ -4,7 +4,6 @@
 
 #include <errno.h>
 #include <unistd.h>
-#include <stdio.h>
 #include <fcntl.h>
 #include <string.h>
 #include <stdlib.h>
@@ -88,11 +87,8 @@ int filename__read_debuglink(const char *filename __maybe_unused,
  */
 int filename__read_build_id(const char *filename, struct build_id *bid)
 {
-	FILE *fp;
-	int ret = -1;
+	int fd, ret = -1;
 	bool need_swap = false, elf32;
-	u8 e_ident[EI_NIDENT];
-	int i;
 	union {
 		struct {
 			Elf32_Ehdr ehdr32;
@@ -103,28 +99,27 @@ int filename__read_build_id(const char *filename, struct build_id *bid)
 			Elf64_Phdr *phdr64;
 		};
 	} hdrs;
-	void *phdr;
-	size_t phdr_size;
-	void *buf = NULL;
-	size_t buf_size = 0;
+	void *phdr, *buf = NULL;
+	ssize_t phdr_size, ehdr_size, buf_size = 0;
 
-	fp = fopen(filename, "r");
-	if (fp == NULL)
+	fd = open(filename, O_RDONLY);
+	if (fd < 0)
 		return -1;
 
-	if (fread(e_ident, sizeof(e_ident), 1, fp) != 1)
+	if (read(fd, hdrs.ehdr32.e_ident, EI_NIDENT) != EI_NIDENT)
 		goto out;
 
-	if (memcmp(e_ident, ELFMAG, SELFMAG) ||
-	    e_ident[EI_VERSION] != EV_CURRENT)
+	if (memcmp(hdrs.ehdr32.e_ident, ELFMAG, SELFMAG) ||
+	    hdrs.ehdr32.e_ident[EI_VERSION] != EV_CURRENT)
 		goto out;
 
-	need_swap = check_need_swap(e_ident[EI_DATA]);
-	elf32 = e_ident[EI_CLASS] == ELFCLASS32;
+	need_swap = check_need_swap(hdrs.ehdr32.e_ident[EI_DATA]);
+	elf32 = hdrs.ehdr32.e_ident[EI_CLASS] == ELFCLASS32;
+	ehdr_size = (elf32 ? sizeof(hdrs.ehdr32) : sizeof(hdrs.ehdr64)) - EI_NIDENT;
 
-	if (fread(elf32 ? (void *)&hdrs.ehdr32 : (void *)&hdrs.ehdr64,
-		  elf32 ? sizeof(hdrs.ehdr32) : sizeof(hdrs.ehdr64),
-		  1, fp) != 1)
+	if (read(fd,
+		 (elf32 ? (void *)&hdrs.ehdr32 : (void *)&hdrs.ehdr64) + EI_NIDENT,
+		 ehdr_size) != ehdr_size)
 		goto out;
 
 	if (need_swap) {
@@ -138,14 +133,18 @@ int filename__read_build_id(const char *filename, struct build_id *bid)
 			hdrs.ehdr64.e_phnum = bswap_16(hdrs.ehdr64.e_phnum);
 		}
 	}
-	phdr_size = elf32 ? hdrs.ehdr32.e_phentsize * hdrs.ehdr32.e_phnum
-			  : hdrs.ehdr64.e_phentsize * hdrs.ehdr64.e_phnum;
+	if ((elf32 && hdrs.ehdr32.e_phentsize != sizeof(Elf32_Phdr)) ||
+	    (!elf32 && hdrs.ehdr64.e_phentsize != sizeof(Elf64_Phdr)))
+		goto out;
+
+	phdr_size = elf32 ? sizeof(Elf32_Phdr) * hdrs.ehdr32.e_phnum
+			  : sizeof(Elf64_Phdr) * hdrs.ehdr64.e_phnum;
 	phdr = malloc(phdr_size);
 	if (phdr == NULL)
 		goto out;
 
-	fseek(fp, elf32 ? hdrs.ehdr32.e_phoff : hdrs.ehdr64.e_phoff, SEEK_SET);
-	if (fread(phdr, phdr_size, 1, fp) != 1)
+	lseek(fd, elf32 ? hdrs.ehdr32.e_phoff : hdrs.ehdr64.e_phoff, SEEK_SET);
+	if (read(fd, phdr, phdr_size) != phdr_size)
 		goto out_free;
 
 	if (elf32)
@@ -153,8 +152,8 @@ int filename__read_build_id(const char *filename, struct build_id *bid)
 	else
 		hdrs.phdr64 = phdr;
 
-	for (i = 0; i < elf32 ? hdrs.ehdr32.e_phnum : hdrs.ehdr64.e_phnum; i++) {
-		size_t p_filesz;
+	for (int i = 0; i < (elf32 ? hdrs.ehdr32.e_phnum : hdrs.ehdr64.e_phnum); i++) {
+		ssize_t p_filesz;
 
 		if (need_swap) {
 			if (elf32) {
@@ -180,8 +179,8 @@ int filename__read_build_id(const char *filename, struct build_id *bid)
 				goto out_free;
 			buf = tmp;
 		}
-		fseek(fp, elf32 ? hdrs.phdr32[i].p_offset : hdrs.phdr64[i].p_offset, SEEK_SET);
-		if (fread(buf, p_filesz, 1, fp) != 1)
+		lseek(fd, elf32 ? hdrs.phdr32[i].p_offset : hdrs.phdr64[i].p_offset, SEEK_SET);
+		if (read(fd, buf, p_filesz) != p_filesz)
 			goto out_free;
 
 		ret = read_build_id(buf, p_filesz, bid, need_swap);
@@ -194,7 +193,7 @@ int filename__read_build_id(const char *filename, struct build_id *bid)
 	free(buf);
 	free(phdr);
 out:
-	fclose(fp);
+	close(fd);
 	return ret;
 }
 
-- 
2.51.0.rc2.233.g662b1ed5c5-goog