commit 3d6890e58caf999f4c83932d27ddf5ec673cf014
Author:     Jacob Welsh <jfw>
AuthorDate: Tue Oct 28 21:23:40 2025 +0000
Commit:     Jacob Welsh <jfw>
CommitDate: Tue Oct 28 21:23:40 2025 +0000

    busybox/testsuite: add tests for grep context buffer leak for which the prior attempted fix caused data loss at 2^31 input lines.
    
    These successfully demonstrate that the original bug was real, that a first attempt of just switching to uint64_t leaves things broken in yet another way, and that the current stronger approach of clearing the buffer on each new file resolves it for all those known cases.

diff --git a/base/busybox/testsuite/grep.tests b/base/busybox/testsuite/grep.tests
index f6d9f21..7a63c5c 100755
--- a/base/busybox/testsuite/grep.tests
+++ b/base/busybox/testsuite/grep.tests
@@ -191,6 +191,40 @@ testing "grep -r on dir/symlink to dir" \
 	"" ""
 rm -Rf grep.testdir
 
+optional FEATURE_GREP_CONTEXT
+mkdir -p grep.testdir
+cat >grep.testdir/a <<EOF
+1
+2
+3xy
+4
+5
+EOF
+
+cat >grep.testdir/b <<EOF
+Ax
+By
+C
+EOF
+
+# Bug originally described in comment on checking for the seemingly impossible (linenum < 1) as "Happens when we go to next file, immediately hit match and try to print prev context... from prev file! Don't do it" (i.e. don't print it).
+testing "grep context buffer leak between files, 1 line" \
+	"grep -B1 x grep.testdir/a grep.testdir/b" \
+	"grep.testdir/a-2\ngrep.testdir/a:3xy\n--\ngrep.testdir/b:Ax\n" \
+	"" ""
+# This variant shows that the match in the next file doesn't have to be immediate.
+testing "grep context buffer leak between files, 2 lines offset 1" \
+	"grep -B2 y grep.testdir/a grep.testdir/b" \
+	"grep.testdir/a-1\ngrep.testdir/a-2\ngrep.testdir/a:3xy\n--\ngrep.testdir/b-Ax\ngrep.testdir/b:By\n" \
+	"" ""
+# This variant comes un-fixed if you naively change linenum to an unsigned type, because the (linenum < 1) test then reduces to (linenum == 0) but wrapping below 0 is possible.
+testing "grep context buffer leak between files, 2 lines at start" \
+	"grep -B2 x grep.testdir/a grep.testdir/b" \
+	"grep.testdir/a-1\ngrep.testdir/a-2\ngrep.testdir/a:3xy\n--\ngrep.testdir/b:Ax\n" \
+	"" ""
+rm -Rf grep.testdir
+SKIP= # end FEATURE_GREP_CONTEXT
+
 # testing "test name" "commands" "expected result" "file input" "stdin"
 #   file input will be file called "input"
 #   test can create a file "actual" instead of writing to stdout